[core] Simplify refType

Author: NMinhNguyen

packages/material-ui-utils/src/refType.js

@@ -1,5 +1,5 @@
 import PropTypes from 'prop-types';
 
-const refType = PropTypes.oneOfType([PropTypes.func, PropTypes.PropTypes.object]);
+const refType = PropTypes.oneOfType([PropTypes.func, PropTypes.object]);
 
 export default refType;