docs(guide/External Resources): add a paid-online course

Closes #15075

docs/content/guide/external-resources.ngdoc

@@ -141,7 +141,7 @@ You can find a larger list of Angular external libraries at [ngmodules.org](http
   [Pluralsite (3 courses)](http://www.pluralsight.com/training/Courses/Find?highlight=true&searchTerm=angularjs),
   [Tuts+](https://tutsplus.com/course/easier-js-apps-with-angular/),
   [lynda.com](http://www.lynda.com/AngularJS-tutorials/Up-Running-AngularJS/133318-2.html),
-  [WintellectNOW (4 lessons)](http://www.wintellectnow.com/Course/Detail/mastering-angularjs)
+  [WintellectNOW (4 lessons)](http://www.wintellectnow.com/Course/Detail/mastering-angularjs),
+  [Packt](https://www.packtpub.com/web-development/angularjs-maintaining-web-applications)
 * **Paid onsite:**
   [angularbootcamp.com](http://angularbootcamp.com/)
-