docs: use `master` instead of `rolling` in contributing.md (#4115)

Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -12,7 +12,7 @@ One of the best ways to begin contributing in a meaningful way is by helping fin
 
 1. Follow the [Installation](https://www.lunarvim.org/docs/installation) guide
 2. Link your fork with the repository `git remote add upstream https://github.com/lunarvim/LunarVim.git`, or use `gh fork`
-3. That's it! You can now `git fetch upstream` and `git rebase [-i] upstream/rolling` to update your branches with the latest contributions.
+3. That's it! You can now `git fetch upstream` and `git rebase [-i] upstream/master` to update your branches with the latest contributions.
 
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