Return super in example Transport subclass (#980)

If called without a callback, the request method returns a Promise, so
when calling into super.request, the result should be returned to
maintain promise behavior.

docs/configuration.asciidoc

@@ -251,7 +251,7 @@ use the usual client code. In such cases, call `super.method`:
 class MyTransport extends Transport {
   request (params, options, callback) {
     // your code
-    super.request(params, options, callback)
+    return super.request(params, options, callback)
   }
 }
 ----