fix(docs): add troubleshooting

docs/site/Calling-other-APIs-and-Web-Services.md

@@ -257,22 +257,18 @@ export class GeoServiceProvider implements Provider<GeoService> {
 }
 ```
 
-In your application, apply
-[ServiceMixin](http://apidocs.loopback.io/@loopback%2fdocs/service-proxy.html#ServiceMixin)
-and use `app.serviceProvider` API to create binding for the geo service proxy.
-
-```ts
-app.serviceProvider(GeoServiceProvider);
-```
+### Troubleshooting
 
-Finally, modify the controller to receive our new service proxy in the
-constructor:
+If you get the error about the
+`app.serviceProvider() function is needed for ServiceBooter`, make sure you
+apply
+[ServiceMixin](http://apidocs.loopback.io/@loopback%2fdocs/service-proxy.html#ServiceMixin)
+to your Application class in the `application.ts`.
 
 ```ts
-export class MyController {
-  @inject('services.GeoService')
-  private geoService: GeoService;
-}
+export class MyLoopBackApplication extends BootMixin(
+  ServiceMixin(RepositoryMixin(RestApplication)),
+)
 ```
 
 Please refer to