misc: Reword CONTRIBUTING.md pre-commit documentation

This is a slight-rewording to make the usage of the pre-commit checks
clearer.

Change-Id: I73f9bfe7143a527c9e495cdf4d51e2aa857b43d5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/63791
Reviewed-by: Jason Lowe-Power <[email protected]>
Maintainer: Jason Lowe-Power <[email protected]>
Tested-by: kokoro <[email protected]>

Author: BobbyRBruce

CONTRIBUTING.md

@@ -143,26 +143,28 @@ Setting up pre-commit
 To help ensure the gem5 style guide is maintained, we use [pre-commit](
 https://pre-commit.com) to run checks on changes to be contributed.
 
-To setup pre-commit, run the following in your gem5 directory.
+To setup pre-commit, run the following in your gem5 directory to install the
+pre-commit and commit message hooks.
 
 ```sh
 pip install pre-commit
 pre-commit install -t pre-commit -t commit-msg
 ```
 
-When you run a `git commit` command the pre-commit will run checks on your
+The hooks are also automatically installed when gem5 is compiled.
+
+When you run a `git commit` command the pre-commit hook will run checks on your
 committed code. The commit will be blocked if a check fails.
 
 The same checks are run as part of Gerrit's CI tests (those required to obtain
 a Verified label, necessary for a change to be accepted to the develop branch).
 Therefore setting up pre-commit in your local gem5 development environment is
 recommended.
 
-You can automatically format your changed code to pass the pre-commit tests by
-running:
+You can automatically format files to pass the pre-commit tests by running:
 
 ```sh
-pre-commit run
+pre-commit run --files <files to format>
 ```
 
 Requirements for change descriptions