Use correct property for index in update challenge

With this commit we use the correct property `index` in the
`bulk-update` challenge instead of the outdated property name
`index_name`. This is a leftover from elastic#36

Relates elastic#44

eventdata/challenges/bulk-update.json

@@ -18,7 +18,7 @@
     {
       "operation": {
         "operation-type": "create-index",
-        "index_name": "elasticlogs",
+        "index": "elasticlogs",
         "settings": {
             {% if index_refresh_interval is defined %}
             "index.refresh_interval": {{ index_refresh_interval | int }},