Fix git-gc usage note

Signed-off-by: Matthias Kestenholz <matthias@spinlock.ch>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-gc.sh

@@ -4,7 +4,7 @@
 #
 # Cleanup unreachable files and optimize the repository.
 
-USAGE='git-gc [--prune]'
+USAGE='[--prune]'
 SUBDIRECTORY_OK=Yes
 . git-sh-setup