Remove fancy quote from a code comment

This isn't always recognized as UTF-8 on all platforms after
copy-pasting.

Fixes #2606
Related to #2539, #1307, #1533

Author: carols10cents

listings/ch04-understanding-ownership/listing-04-03/src/main.rs

@@ -7,7 +7,7 @@ fn main() {
     let x = 5;                      // x comes into scope
 
     makes_copy(x);                  // x would move into the function,
-                                    // but i32 is Copy, so it’s okay to still
+                                    // but i32 is Copy, so it's okay to still
                                     // use x afterward
 
 } // Here, x goes out of scope, then s. But because s's value was moved, nothing