Update source/applications/aggregation.txt

Update the $group usage in the example to use the updated syntax,
as per the group posting below.

https://groups.google.com/d/topic/mongodb-user/1cYch580h0w/discussion

Author: davidmc24

source/applications/aggregation.txt

@@ -143,7 +143,7 @@ command:
      } },
      { $unwind : “$tags” },
      { $group : {
-        _id : { tags : 1 },
+        _id : { tags : "$tags" },
         authors : { $addToSet : “$author” }
      } }
    );