docs(Courses): fix syntax issue in developer guide

The courses section should use commas between links to differentiate the instances of each link

Closes angular#10448

Author: wesleysmyth

docs/content/guide/index.ngdoc

@@ -115,12 +115,12 @@ This is a short list of libraries with specific support and documentation for wo
 ### Courses
 * **Free online:**
   [thinkster.io](http://thinkster.io),
-  [CodeAcademy](http://www.codecademy.com/courses/javascript-advanced-en-2hJ3J/0/1)
+  [CodeAcademy](http://www.codecademy.com/courses/javascript-advanced-en-2hJ3J/0/1),
   [CodeSchool](https://www.codeschool.com/courses/shaping-up-with-angular-js)
 * **Paid online:**
   [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)
+  [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)
 * **Paid onsite:**
   [angularbootcamp.com](http://angularbootcamp.com/)