[example] Update RouteGuide example (#36958)

Simple change to stop writing if `ok` is false in `OnWriteDone()`

Closes #36958

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36958 from yashykt:UpdateRouteGuideExample cd5b066350
PiperOrigin-RevId: 644471350

examples/cpp/route_guide/route_guide_callback_client.cc

@@ -243,7 +243,11 @@ class RouteGuideClient {
         StartRead(&server_note_);
         StartCall();
       }
-      void OnWriteDone(bool /*ok*/) override { NextWrite(); }
+      void OnWriteDone(bool ok) override {
+        if (ok) {
+          NextWrite();
+        }
+      }
       void OnReadDone(bool ok) override {
         if (ok) {
           std::cout << "Got message " << server_note_.message() << " at "