Fixed $group on compound keys

Author: mikoskinen

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.