add index

Author: shuangela

source/atlas-search.txt

@@ -69,10 +69,11 @@ The query also includes a:
    .. input::
       :language: python
 
-      client = pymongo.MongoClient('<connection-string>')
-      result = client['sample_mflix']['movies'].aggregate([
+      client = pymongo.MongoClient("<connection-string>")
+      result = client["sample_mflix"]["movies"].aggregate([
                {
                   "$search": {
+                  "index": "pymongoindex",
                   "phrase": {
                   "path": "title",
                   "query": "new york"