Merge pull request #347 from edx/e0d/typo-fix

fixed incorrect yaml merge source.

playbooks/roles/edxapp/defaults/main.yml

@@ -118,7 +118,7 @@ edxapp_generic_auth_config:  &edxapp_generic_auth
       password: $EDXAPP_MONGO_PASSWORD
       port: $EDXAPP_MONGO_PORT
       user: $EDXAPP_MONGO_USER
-    DOC_STORE_CONFIG: *edxapp_generic_default_modulestore
+    DOC_STORE_CONFIG: *edxapp_generic_default_docstore
   MODULESTORE:
     default: &edxapp_generic_default_modulestore
       ENGINE: 'xmodule.modulestore.mongo.DraftMongoModuleStore'