git submodule: fix usage line

Actually, you have to set the -b option after the add command.
Signed-off-by: NJulien Danjou <julien@danjou.info>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -5,7 +5,7 @@
 # Copyright (c) 2007 Lars Hjemli
 
 USAGE="[--quiet] [--cached] \
-[add <repo> [-b branch] <path>]|[status|init|update [-i|--init] [-N|--no-fetch]|summary [-n|--summary-limit <n>] [<commit>]] \
+[add [-b branch] <repo> <path>]|[status|init|update [-i|--init] [-N|--no-fetch]|summary [-n|--summary-limit <n>] [<commit>]] \
 [--] [<path>...]|[foreach <command>]|[sync [--] [<path>...]]"
 OPTIONS_SPEC=
 . git-sh-setup