[C#] Code Snippet for Atlas Search Autocomplete doesn't work (#177)

* add note about search index

Author: rachel-mack

source/fundamentals/atlas-search.txt

@@ -106,6 +106,10 @@ collection using the string "Gib" in the ``make`` field.
    :language: csharp
    :dedent:
 
+.. note::
+   
+   If the field you are searching on is indexed by a search index, you must pass the index name to the ``Autocomplete`` call. If a search index does not exist, the default index is used.
+
 The search returns the following document:
 
 .. code-block:: json

source/includes/fundamentals/code-examples/atlas-search/AtlasSearchExamples.cs

@@ -29,7 +29,7 @@ public static List<Guitar> AutocompleteSearch()
         // Finds documents with a "make" value that contains the string fragment "Gib"
         // start-autocomplete-search
         var result = guitarsCollection.Aggregate()
-            .Search(Builders<Guitar>.Search.Autocomplete(g => g.Make, "Gib"))
+            .Search(Builders<Guitar>.Search.Autocomplete(g => g.Make, "Gib"), indexName: "guitarmakes")
             .ToList();
         // end-autocomplete-search