Fix example in documentation

[ivantsepp]

In the docs, the example for high volume is

  google_pubsub {
    project_id => "my_project"
    topic => "my_topic"
    json_key_file => "service_account_key.json"

    # Options for configuring the upload
    message_count_threshold => 10000
    delay_threshold_secs => 10
    request_byte_threshold => 5MB
  }
}

But the docs say the max message_count_threshold is 1000. (https://github.com/googleapis/google-cloud-java/blob/dc2672ad1838a8d15a66e1f1697c97c84e4e0933/google-cloud-clients/google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/Publisher.java#L113). So let's fix this example.

Also, 5MB isn't a valid value and request_byte_threshold expects an integer. So, fix this in the example as well as the docs where it shows the default. (I made the mistake of keeping the underscores in the integer since it's valid in Ruby).

This should also address https://github.com/logstash-plugins/logstash-output-google_pubsub/issues/16