submodule: fix submodule--helper clone usage

git submodule--helper clone usage stated that paths were added after the
[--] argument. The actual implementation required use of --path argument
and only supports one path at a time. Update the usage string to match
the current implementation.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -187,7 +187,7 @@ static int module_clone(int argc, const char **argv, const char *prefix)
 	const char *const git_submodule_helper_usage[] = {
 		N_("git submodule--helper clone [--prefix=<path>] [--quiet] "
 		   "[--reference <repository>] [--name <name>] [--url <url>]"
-		   "[--depth <depth>] [--] [<path>...]"),
+		   "[--depth <depth>] [--path <path>]"),
 		NULL
 	};