git-branch.txt: document -f correctly
git-branch.txt: document -f correctly

'git branch -f a b' resets a to b when a exists, rather then deleting a.
Say so in the documentation.

Signed-off-by: Michael J Gruber <46f1a0bd5592a2f9244ca321b129902a06b53e03@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git