docs: Fix rustfmt install steps.

Fixes #2080

Author: emilio

CONTRIBUTING.md

@@ -100,7 +100,7 @@ compared. Make sure you have `rustfmt` up to date:
 
 ```
 $ rustup update nightly
-$ rustup run nightly cargo install -f rustfmt-nightly
+$ rustup component add rustfmt --toolchain nightly
 ```
 
 Note: running `cargo test` from the root directory of `bindgen`'s repository does not

tests/tests.rs

@@ -49,7 +49,7 @@ The latest `rustfmt` is required to run the `bindgen` test suite. Install
 `rustfmt` with:
 
     $ rustup update nightly
-    $ rustup run nightly cargo install -f rustfmt-nightly
+    $ rustup component add rustfmt --toolchain nightly
 "
             );
         }