fix react proptypes warning

Author: johnwils

lib/components/Modal.js

@@ -38,7 +38,7 @@ var Modal = createReactClass({
       overlay: PropTypes.object
     }),
     portalClassName: PropTypes.string,
-    bodyOpenClassName: React.PropTypes.string,
+    bodyOpenClassName: PropTypes.string,
     appElement: PropTypes.instanceOf(SafeHTMLElement),
     onAfterOpen: PropTypes.func,
     onRequestClose: PropTypes.func,