Fix small issue in ReactContext comment

Noticed this the other day and meant to fix it.

src/core/ReactContext.js

@@ -40,7 +40,7 @@ var ReactContext = {
    * A typical use case might look like
    *
    *  render: function() {
-   *    var children = ReactContext.withContext({foo: 'foo'} () => (
+   *    var children = ReactContext.withContext({foo: 'foo'}, () => (
    *
    *    ));
    *    return <div>{children}</div>;