Adds missing comma

This will result in an error if people copy this config.

Thanks!

Author: chadfennell

README.md

@@ -99,7 +99,7 @@ config :my_app, MyApp.ElasticsearchCluster,
 
       # Likewise, wait a given period between posting pages to give
       # Elasticsearch time to catch up.
-      bulk_wait_interval: 15_000 # 15 seconds
+      bulk_wait_interval: 15_000, # 15 seconds
 
       # By default bulk indexing uses the "create" action. To allow existing
       # documents to be replaced, use the "index" action instead.