Update docs for Amazon ElasticSearch service as backend.

This change moves the "connection_class" key into "OPTIONS", resolving error
``AttributeError: 'AWS4Auth' object has no attribute 'encode'``.

docs/topics/search/backends.rst

@@ -183,7 +183,7 @@ The Elasticsearch backend is compatible with `Amazon Elasticsearch Service`_, bu
 
   WAGTAILSEARCH_BACKENDS = {
       'default': {
-          'BACKEND': 'wagtail.wagtailsearch.backends.elasticsearch',
+          'BACKEND': 'wagtail.wagtailsearch.backends.elasticsearch2',
           'INDEX': 'wagtail',
           'TIMEOUT': 5,
           'HOSTS': [{
@@ -193,7 +193,9 @@ The Elasticsearch backend is compatible with `Amazon Elasticsearch Service`_, bu
               'verify_certs': True,
               'http_auth': AWS4Auth('ACCESS_KEY', 'SECRET_KEY', 'REGION', 'es'),
           }],
+          'OPTIONS': {
               'connection_class': RequestsHttpConnection,
+          },
       }
   }