Clarify the valid usage printed on invalid usage

main.c

@@ -334,7 +334,7 @@ int main(int argc, char* argv[]) {
     wl_display_roundtrip(dpy);
   } else {
     ASSERT(argc >= 1, "Invalid exec!");
-    printf("Usage: %s ?|<identifier>\n", argv[0]);
+    printf("Usage: %s [?|<identifier>]\n", argv[0]);
     exit(-1);
   }