Fix rapid7#4098 - False negative check for script_mvel_rce

Fix rapid7#4098, thanks @arnaudsoullie

Author: wchen-r7

modules/exploits/multi/elasticsearch/script_mvel_rce.rb

@@ -127,7 +127,7 @@ def parse_result(res)
     end
 
     begin
-      result = json['hits']['hits'][0]['fields']['msf_result'][0]
+      result = json['hits']['hits'][0]['fields']['msf_result']
     rescue
       return nil
     end