Fixes node client section name on Java API Doc

The links to the "node client" section on the [client java-api doc](http://www.elasticsearch.org/guide/en/elasticsearch/client/java-api/master/client.html) don't work due to a minor typo in the references.
btw, if you just want to link to a section, you can use `<<node-client>>` and it will generate the text from the title.  I'll fix this while merging.  thanks!