merge: minor correction

Author: Sam Kleinman

source/reference/aggregation/group.txt

@@ -20,7 +20,7 @@ $group (aggregation)
 
    .. code-block:: javascript
 
-      { _id : { author: 1, pageViews: 1, posted: 1 } }
+      { _id : { author: '$author', pageViews: '$pageViews', posted: '$posted' } }
 
    With the exception of the ``_id`` field, :pipeline:`$group` cannot
    output nested documents.