Docs: Move id in the java-api (#40748)

Moves the id of the preface in the java-api so it is compatible with
both AsciiDoc and Asciidoctor. As it stands now we apply the id that we
want for the preface to the book itself which is strange and only works
with AsciiDoc.

Author: nik9000

docs/java-api/index.asciidoc

@@ -1,7 +1,7 @@
-[[java-api]]
 = Java API
 :ref: http://www.elastic.co/guide/en/elasticsearch/reference/2.4
 
+[[java-api]]
 [preface]
 == Preface
 This section describes the Java API that elasticsearch provides. All