misc: Updated CONTRIBUTING.md: 'master' -> 'stable'

The `master` branch is now the `stable` branch. This commit updates
CONTRIBUTING.md accordingly.

Change-Id: Ic5231eda336520e8a7260efac6474b4f0af08c37
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35457
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed