Add example of git cherry-pick for backporting bug fixes

The cherry-pick section mentions backporting bug fixes but doesn't show a concrete example.

Location https://docs.gitlab.com/ee/topics/git/advanced.html

Current Text "Use it to: Backport bug fixes from the default branch to previous release branches."

Suggested Fix Add an example: "For example, if you fixed a bug on your main branch with commit abc123, you can apply just that fix to your release-v1 branch with: git checkout release-v1 then git cherry-pick abc123"

Why This Helps Gives users a practical example they can follow.

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