Fix Lucene version in the docs.

docs/reference/index.asciidoc

@@ -9,7 +9,7 @@ release-state can be: released | prerelease | unreleased
 //////////
 
 :release-state:  unreleased
-:lucene_version: 6.3.4
+:lucene_version: 6.3.0
 :branch:         master
 :jdk:            1.8.0_73
 :defguide:       https://www.elastic.co/guide/en/elasticsearch/guide/master