Fix misleading comment in exec_js_renderer
Fix misleading comment in exec_js_renderer

As of 41721c5374925384adc8679aa007789b598de969, `exec_js_renderer` now needs `ReactRailsUJS` to appear in the global context, not `ReactDOM`.

reactjs/react-rails,reactjs/react-rails,reactjs/react-rails,reactjs/react-rails