docs: update pubsub example

[anguillanneuf]

The existing typed background function example has two problems:

1. Classes in the default package cannot be imported by classes in packages. This applies to com.example.PubSubBackground.PubSubMessage.
2. pubSubMessage.messageId will always return null. Instead, the example should print event ID, which is populated by the message ID.
   

   functions-framework-java/invoker/core/src/main/java/com/google/cloud/functions/invoker/Event.java

   Line 70 in 9a576de

   .setEventId(message.get("messageId").getAsString())

I published some instructions for testing functions-framework-java with the Pub/Sub emulator in my personal repo. I was hoping to include them here after the next release of the library. See #104.

[matthewrobertson]

Looks good, thanks!