Update source/tutorial/aggregation-examples.txt

Use the same collection name in all examples.
Previously, two were "zipcodes" and one was "zipcode".
Now all will be "zipcodes".

Author: davidmc24

source/tutorial/aggregation-examples.txt

@@ -133,7 +133,7 @@ following aggregation operation:
 
 .. code-block:: javascript
 
-   db.zipcode.aggregate({ $group :
+   db.zipcodes.aggregate({ $group :
                           { _id : { state : "$state", city : "$city" },
                             pop : { $sum : "$pop" } } },
                         { $group :