Update source/reference/operator/centerSphere.txt

Code didn't use $within which results in an error.

Author: dlouwers

source/reference/operator/centerSphere.txt

@@ -17,7 +17,7 @@ $centerSphere
 
    .. code-block:: javascript
 
-      db.collection.find( { loc: { $centerSphere: { [0,0], 10 / 3959 } } } )
+      db.collection.find( { loc: { $within: { $centerSphere: { [0,0], 10 / 3959 } } } } )
 
    This query will return all documents within a 10 mile radius of
    ``[0,0]`` using a spherical geometry to calculate distances.