docs(guide/index): replace "shold" to "should"

Replace "shold" to "should"

Closes angular#6216

Author: Stéphane Reynaud

docs/content/guide/index.ngdoc

@@ -13,7 +13,7 @@ Everything you need to know about AngularJS
 
 * {@link tutorial/index Official AngularJS Tutorial}
 * [10 Reasons Why You Should Use AngularJS](http://www.sitepoint.com/10-reasons-use-angularjs/)
-* [10 Reasons Why Developers Shold Learn AngularJS](http://wintellect.com/blogs/jlikness/10-reasons-web-developers-should-learn-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)