fix bridge producerWindowSize

According to the documentation, a property named producer-window-size should exist for bridge configuration. This is documented both here: https://activemq.apache.org/artemis/docs/1.2.0/core-bridges.html and here: https://activemq.apache.org/artemis/docs/1.2.0/configuration-index.html#bridge-type.

When I tried adding the <producer-window-size> element to my broker.xml I get an error saying that it is invalid. Looking at the XSD I have confirmed that it is not listed there.