Link Fix / https://developer.shotgunsoftware.com/tk-core/platform.html#applications

Hey there! I was reading up on the documentation surrounding apps in the _Shotgun Toolkit_ and I noticed that the link in [the applications introduction](https://developer.shotgunsoftware.com/tk-core/platform.html#applications) part was broken. I tried looking for the correct link, but couldn't immediately find it. Perhaps this could be fixed?