updated concepts/Upgrading/Upgrading.md

Fixed the 'done()-vs-exec()' link in the 'Contents' -> 'jump too...' section. Fixed by encoding the parentheses as '%28&29' instead of '()'. 

To make sure this fix works, I tested it out by compiling the new Markdown and verifying the `href=""` pointed to the correct link. Then I edited the http://sailsjs.org/#/documentation/concepts/Upgrading page with Firebug with the correct markdown created for the `href=""` part and then the link worked when I clicked it.