Fix mistake in docker rmi command (dotnet#2042)

It had the container name, not the image name.

docs/csharp/tutorials/microservices.md

@@ -459,7 +459,7 @@ docker rm hello-docker
 If you want to remove unused images from your machine, you use this command:
 
 ```console
-docker rmi hello-docker
+docker rmi weather-microservice
 ```
 
 ## Conclusion