"branch -d" can remove more than one branches

Since 03feddd6 (git-check-ref-format: reject funny ref names, 2005-10-13),
"git branch -d" can take more than one branch names to remove.

The documentation was correct, but the usage string was not.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -19,7 +19,7 @@
 static const char * const builtin_branch_usage[] = {
 	"git branch [options] [-r | -a] [--merged | --no-merged]",
 	"git branch [options] [-l] [-f] <branchname> [<start-point>]",
-	"git branch [options] [-r] (-d | -D) <branchname>",
+	"git branch [options] [-r] (-d | -D) <branchname>...",
 	"git branch [options] (-m | -M) [<oldbranch>] <newbranch>",
 	NULL
 };