Properly close snippet pubsub_subscriber_custom_credentials (#3575)

snippets.go fails and exits due to `snippet pubsub_subscriber_custom_credentials` not being closed.

```
$ go run utilities/snippets.go .
snippet: [START
google-cloud-examples/src/main/java/com/google/cloud/examples/pubsub/snippets/SubscriberSnippets.java]:178 snippet "pubsub_subscriber_custom_credentials" not closed
exit status 1
```

Rename the 2nd `START` to `END` to correct the typo.

Author: nithinsujir

google-cloud-examples/src/main/java/com/google/cloud/examples/pubsub/snippets/SubscriberSnippets.java

@@ -184,7 +184,7 @@ private Subscriber createSubscriberWithCustomCredentials() throws Exception {
         Subscriber.newBuilder(subscriptionName, receiver)
             .setCredentialsProvider(credentialsProvider)
             .build();
-    // [START pubsub_subscriber_custom_credentials]
+    // [END pubsub_subscriber_custom_credentials]
     return subscriber;
   }