correct field name for routing (elastic#758)

`_routing` is correct field name for `routing` in every bulk item action

Author: sumit0k

docs/helpers.rst

@@ -28,7 +28,7 @@ in several formats. The most common one is the same as returned by
  '_index': 'index-name',
  '_type': 'document',
  '_id': 42,
- 'routing': 5,
+ '_routing': 5,
  'pipeline': 'my-ingest-pipeline',
  '_source': {
  "title": "Hello World!",
@@ -43,7 +43,7 @@ from the doc and use the rest as the document data:
 
  {
  "_id": 42,
- "routing": 5,
+ "_routing": 5,
  "title": "Hello World!",
  "body": "..."
  }