Fixed incorrect screencast link

The titles of screencasts linked properly to the screencast page (grails.org/screencast/22, for example), but the "read more" link pointed at tutorials (grails.org/tutorial/22), not screencasts. To replicate this, check out this page:
http://grails.org/screencasts
and check out the difference between the link of the screencast title and the link of the "read more."

My first pull request; please let me know if you need any other info. Cheers!