docs(guide): grammar fix

use of the article, both definite and indefinite, is not optional

Closes #5981

Author: gjvc

docs/content/guide/index.ngdoc

@@ -16,7 +16,7 @@ Everything you need to know about AngularJS
 * [10 Reasons Why Developers Should Learn AngularJS](http://wintellect.com/blogs/jlikness/10-reasons-web-developers-should-learn-angularjs)
 * [Design Principles of AngularJS (video)](https://www.youtube.com/watch?v=HCR7i5F5L8c)
 * [Fundamentals in 60 Minutes (video)](http://www.youtube.com/watch?v=i9MHigUZKEM)
-* [For folks with jQuery background](http://stackoverflow.com/questions/14994391/how-do-i-think-in-angularjs-if-i-have-a-jquery-background)
+* [For folks with a jQuery background](http://stackoverflow.com/questions/14994391/how-do-i-think-in-angularjs-if-i-have-a-jquery-background)
 
 ## Core Concepts