Auto merge of rust-lang#6470 - xFrednet:0000-rename-good-first-issue-in-docs, r=flip1995

Renamed the good first issue label in CONTRIBUTING.md

Follow up from rust-lang#6468

Grep found some more references to the old `good first issue`.

[CONTRIBUTING.md rendered](https://github.com/xFrednet/rust-clippy/blob/0000-rename-good-first-issue-in-docs/CONTRIBUTING.md)

<details>
<summary>grep output</summary>

```
$ grep -Ri "good first issue" rust-clippy/
rust-clippy/.git/COMMIT_EDITMSG:Renamed the good first issue label in CONTRIBUTING.md
rust-clippy/.git/logs/HEAD:896d82f7ff64644656bda7a4ed8bbd55ca3b7619 1f58c2b xFrednet <[email protected]> 1608326295 +0000     commit: Renamed the good first issue label for rustbot
rust-clippy/.git/logs/HEAD:9be704584f05e5a6c3ba2708590f98c1f261d19a ced54f2 xFrednet <[email protected]> 1608329602 +0000     commit: Renamed the good first issue label in CONTRIBUTING.md
rust-clippy/.git/logs/refs/heads/0000-rename-good-first-issue-in-docs:896d82f7ff64644656bda7a4ed8bbd55ca3b7619 1f58c2b xFrednet <[email protected]> 1608326295 +0000  commit: Renamed the good first issue label for rustbot
rust-clippy/.git/logs/refs/heads/0000-rename-good-first-issue-in-docs:9be704584f05e5a6c3ba2708590f98c1f261d19a ced54f2 xFrednet <[email protected]> 1608329602 +0000  commit: Renamed the good first issue label in CONTRIBUTING.md
rust-clippy/CONTRIBUTING.md:Some issues are easier than others. The [`good first issue`] label can be used to find the easy issues.
rust-clippy/CONTRIBUTING.md:[`E-medium`] issues are generally pretty easy too, though it's recommended you work on an [`good first issue`]
rust-clippy/CONTRIBUTING.md:[`good first issue`]: https://github.com/rust-lang/rust-clippy/labels/good%20first%20issue
```
</details>

---

changelog: None

r? `@flip1995`

Author: bors

CONTRIBUTING.md

@@ -49,7 +49,7 @@ first read the [Basics docs](doc/basics.md).**
 All issues on Clippy are mentored, if you want help with a bug just ask
 @Manishearth, @flip1995, @phansch or @yaahc.
 
-Some issues are easier than others. The [`good first issue`] label can be used to find the easy issues.
+Some issues are easier than others. The [`good-first-issue`] label can be used to find the easy issues.
 If you want to work on an issue, please leave a comment so that we can assign it to you!
 
 There are also some abandoned PRs, marked with [`S-inactive-closed`].
@@ -68,7 +68,7 @@ To figure out how this syntax structure is encoded in the AST, it is recommended
 Usually the lint will end up to be a nested series of matches and ifs, [like so][deep-nesting].
 But we can make it nest-less by using [if_chain] macro, [like this][nest-less].
 
-[`E-medium`] issues are generally pretty easy too, though it's recommended you work on an [`good first issue`]
+[`E-medium`] issues are generally pretty easy too, though it's recommended you work on an [`good-first-issue`]
 first. Sometimes they are only somewhat involved code wise, but not difficult per-se.
 Note that [`E-medium`] issues may require some knowledge of Clippy internals or some
 debugging to find the actual problem behind the issue.
@@ -77,7 +77,7 @@ debugging to find the actual problem behind the issue.
 lot of methods that are useful, though one of the most useful would be `expr_ty` (gives the type of
 an AST expression). `match_def_path()` in Clippy's `utils` module can also be useful.
 
-[`good first issue`]: https://github.com/rust-lang/rust-clippy/labels/good%20first%20issue
+[`good-first-issue`]: https://github.com/rust-lang/rust-clippy/labels/good-first-issue
 [`S-inactive-closed`]: https://github.com/rust-lang/rust-clippy/pulls?q=is%3Aclosed+label%3AS-inactive-closed
 [`T-AST`]: https://github.com/rust-lang/rust-clippy/labels/T-AST
 [`T-middle`]: https://github.com/rust-lang/rust-clippy/labels/T-middle