worktree: say that "add" takes an arbitrary commit in short-help

c4738aed ("worktree: add can be created from any commit-ish",
2017-11-26) taught "git worktree add" to start a new worktree
with an arbitrary commit-ish checked out, not limited to a tip
of a branch.

"git worktree --help" was updated to describe this, but we forgot to
update "git worktree -h".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/worktree.c

@@ -14,7 +14,7 @@
 #include "worktree.h"
 static const char * const worktree_usage[] = {
-	N_("git worktree add [<options>] <path> [<branch>]"),
+	N_("git worktree add [<options>] <path> [<commit-ish>]"),
 	N_("git worktree list [<options>]"),
 	N_("git worktree lock [<options>] <path>"),
 	N_("git worktree prune [<options>]"),