[DOCS] Fixed an out of sequence header in the Groovy docs

Author: clintongormley

docs/groovy-api/index.asciidoc

@@ -17,7 +17,7 @@ manner. The execution options for each API follow a similar manner and
 covered in <<anatomy>>.
 
 
-==== Maven Repository
+=== Maven Repository
 
 The Groovy API is hosted on
 http://search.maven.org/#search%7Cga%7C1%7Ca%3A%22elasticsearch-client-groovy%22[Maven