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 <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed