[Docs] Update reference for git flow extension repository

[ChrisJStone]

On the Git Flow documentation page the linked repository for gitflow has had no commits since September 25 2012. The currently maintained fork of git-flow is at gitflow-cjs

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. After 30 days from now, it will be closed if no further activity occurs.

[github-actions]

This issue was closed because it has been stalled for 30 days with no activity. Thank you for your contributions