Fix typo

Author: biochimia

source/reference/operator/regex.txt

@@ -67,7 +67,7 @@ $regex
 
    .. code-block:: javascript
 
-      db.collection.find( { field: $regex: /acme.*corp/i, $nin: [ 'acmeblahcorp' } );
+      db.collection.find( { field: { $regex: /acme.*corp/i, $nin: [ 'acmeblahcorp' ] } );
 
    This expression returns all instances of ``field`` in
    ``collection`` that match the case insensitive regular expression