website: Replace master with stable in the getting started page

The master branch is an old unused branch and users might
mistakenly check it out

Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Change-Id: I755393190070125b8e8356e85d01a13d94587b4b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/53783
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
diff --git a/_pages/getting_started.md b/_pages/getting_started.md
index d7270d6..983639d 100644
--- a/_pages/getting_started.md
+++ b/_pages/getting_started.md
@@ -59,23 +59,23 @@
 
 ### What version of gem5 should I use?
 
-The gem5 git repository has two branches: `develop` and `master`. The `develop`
+The gem5 git repository has two branches: `develop` and `stable`. The `develop`
 branch contains the very latest gem5 changes **but is not stable**. It is
 frequently updated. **The `develop` branch should only be used when
 contributing the the gem5 project** (please see our [Contributing Guide](
 /contributing) for more information on how to submit code to gem5).
 
-The master branch contains stable gem5 code. The HEAD of the master branch
+The stable branch contains stable gem5 code. The HEAD of the stable branch
 points towards the latest gem5 release. We would advise researchers use the
 latest stable release of gem5 and report which version was used when publishing
 results (use `git describe` to see latest gem5 release version number).
 
 If replicating previous work, please find which version of gem5 was used. This
-version should be tagged on the `master` branch and can thereby be checked-out
+version should be tagged on the `stable` branch and can thereby be checked-out
 on a new branch using `git checkout -b {branch} {version}`.
 E.g., to checkout `v19.0.0` on a new branch called `version19`:
 `git checkout -b version19 v19.0.0`. A complete list of released gem5
-versions can be determined by executing `git tag` on the `master` branch.
+versions can be determined by executing `git tag` on the `stable` branch.
 
 ### How should I cite gem5?