submodule--helper: fix typo in is_active error message

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -1163,7 +1163,7 @@ static int absorb_git_dirs(int argc, const char **argv, const char *prefix)
 static int is_active(int argc, const char **argv, const char *prefix)
 {
 	if (argc != 2)
-		die("submodule--helper is-active takes exactly 1 arguments");
+		die("submodule--helper is-active takes exactly 1 argument");
 
 	gitmodules_config();