Fix --target parameter for pip install

Author: nicoddemus

_pytest/vendored_packages/README.md

@@ -6,8 +6,8 @@ package, please see [this issue](https://github.com/pytest-dev/pytest/issues/944
 To update the current version, execute:
 
 ```
-$ pip install pluggy==<version> --no-compile --target-dir=_pytest/vendored_packages
+$ pip install pluggy==<version> --no-compile --target=_pytest/vendored_packages
 ```
 
 And commit the modified files. The `pluggy-<version>.dist-info` directory 
-created by `pip` should be ignored.
+created by `pip` should be ignored.