worktree: usage: denote <branch> as optional with 'add'

Although 1eb07d8 (worktree: add: auto-vivify new branch when
<branch> is omitted, 2015-07-06) updated the documentation when
<branch> became optional, it neglected to update the in-code
usage message. Fix this oversight.

Reported-by: ch3cooli@gmail.com
Signed-off-by: Sidhant Sharma <tigerkid001@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/worktree.c

@@ -7,7 +7,7 @@
 #include "sigchain.h"
 
 static const char * const worktree_usage[] = {
-	N_("git worktree add [<options>] <path> <branch>"),
+	N_("git worktree add [<options>] <path> [<branch>]"),
 	N_("git worktree prune [<options>]"),
 	NULL
 };