Add {username} to helloworld-go service definition (#2123)

This commit updates the helloworld-go README.md, step 4
("Recreating the sample code" section), and adds {username}
to the example yaml which is mentioned in the description.

docs/eventing/samples/helloworld/helloworld-go/README.md

@@ -163,7 +163,7 @@ cd knative-docs/docs/eventing/samples/helloworld/helloworld-go
         spec:
           containers:
             - name: helloworld-go
-              image: docker.io/akashv/helloworld-go
+              image: docker.io/{username}/helloworld-go
 
     ---