Minor fix to the doc about XContentFactory's package

[mingfai]

At http://www.elasticsearch.com/docs/elasticsearch/java_api/index/,
import static org.elasticsearch.util.xcontent.XContentFactory.*;"

for the current release, the package is moved and the following should be used:

import static org.elasticsearch.common.xcontent.XContentFactory.*;

[kimchy]

right, will commit a fix to the docs (thats another repo).