DOCS-1151: correction to $within and operation

Author: Sam Kleinman

source/core/geospatial-indexes.txt

@@ -276,8 +276,8 @@ with a radius of ``100`` miles:
 
 .. code-block:: javascript
 
-   db.places.find( { loc: { $centerSphere: [ [ -74, 40.74 ] ,
-                                           100 / 3963.192 ] } } )
+   db.places.find( { loc: { $within: { $centerSphere: [ [ -74, 40.74 ] ,
+                                                        100 / 3963.192 ] } } } )
 
 You may also use the ``distanceMultiplier`` option to the
 :dbcommand:`geoNear` to convert radians in the :program:`mongod`