builtin/submodule--helper: fix usage string for 'update-clone'

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -1874,7 +1874,7 @@ static int update_clone(int argc, const char **argv, const char *prefix)
 	};
 
 	const char *const git_submodule_helper_usage[] = {
-		N_("git submodule--helper update_clone [--prefix=<path>] [<path>...]"),
+		N_("git submodule--helper update-clone [--prefix=<path>] [<path>...]"),
 		NULL
 	};
 	suc.prefix = prefix;