clarify $elemMatch projection example

see http://stackoverflow.com/questions/12274957/does-elemmatch-not-work-with-a-new-mongodb-2-2/12286852#12286852
and Automattic/mongoose#1085

Author: aheckmann

source/reference/projection/elemMatch.txt

@@ -35,9 +35,8 @@ $elemMatch (projection)
 
       .. code-block:: javascript
 
-         db.students.find( { zipcode: 63109 },
-                           { _id: 0,
-                             dependents: { $elemMatch: { school: 102 } } } )
+         var projection = { _id: 0, dependents: { $elemMatch: { school: 102 }}};
+         db.students.find( { zipcode: 63109 }, projection);
 
       The query would return all documents where the value of the
       ``zipcode`` field is ``63109``, while the projection excludes