[doc] fix master build

The `:ref:` link in java-api doc is connected to `current` version which is at this time `1.6`.
This commit patch this.

That being said, we might have to change it again once master will become `current` doc.

docs/java-api/index.asciidoc

@@ -1,6 +1,6 @@
 [[java-api]]
 = Java API
-:ref: http://www.elastic.co/guide/en/elasticsearch/reference/current
+:ref: http://www.elastic.co/guide/en/elasticsearch/reference/master
 
 [preface]
 == Preface