Fixing wrong link in 'Ruby on Rails 3.0 Release Notes' [ci skip]

The link 'http://magicscalingsprinkles.wordpress.com/2010/01/28/why-i-wrote-arel/' points to a page that does not exist anymore.

This is the last snapshot from archive.org to the original article:

https://web.archive.org/web/20120718093140/http://magicscalingsprinkles.wordpress.com/2010/01/28/why-i-wrote-arel/