Update 45_Partial_update.asciidoc

If we do not add  "lang":"groovy",  we will get the error : {
   "error": "ElasticsearchIllegalArgumentException[failed to execute script]; nested: ScriptException[dynamic scripting for [mvel] disabled]; ",
   "status": 400
}
If we include lang parameter, and give value as, (lang=groovy) we do not get the issue 

http://stackoverflow.com/questions/25345260/why-am-i-getting-an-error-in-my-elasticsearch-query-after-upgrading-to-1-3-2