Update WatchMissingNodeModulesPlugin.js

This link is dead: 
// See facebook#186.

Author: tnrich

packages/react-dev-utils/WatchMissingNodeModulesPlugin.js

@@ -7,7 +7,6 @@
 
 // This Webpack plugin ensures `npm install <library>` forces a project rebuild.
 // We’re not sure why this isn't Webpack's default behavior.
-// See https://github.com/facebookincubator/create-react-app/issues/186.
 
 'use strict';