Add --git-arg to README.md
Add --git-arg to README.md

This was previously overlooked.  Fix it.

Signed-off-by: Kevin Locke <ffb4761cba839470133bee36aeb139f58d7dbaa9@kevinlocke.name>

kevinoid/git-branch-is,kevinoid/git-branch-is,kevinoid/git-branch-is