modified StreamSerializer documentation so the XML example matches the l...

I modified the StreamSerializer documentation to make the XML example matches the latest XSD (http://hazelcast.com/schema/spring/hazelcast-spring-3.3.xsd)

hazelcast-spring xsds (versions 3.0-3.3) seem to disallow text nodes inside of the serializer tag, instead exposing the attributes 'implementation' and 'class-name'

I updated the docs to use the class-name attribute instead of the inner text

I tried looking to see which XSD previously allowed the text node, but I couldn't find it, leading me to be curious of whether I am interpreting the XSD incorrectly (and the current doc is actually correct) or if the current doc was always wrong. Some insight on this would be appreciated, thanks!