Fix some issues in 4.2 release notes.

Hi,

I removed duplicate line in Railties section (first occurence at line 121).

And I changed the documentation URL by [http://edgeapi.rubyonrails.org/](http://edgeapi.rubyonrails.org/) because the URL [http://api.rubyonrails.org/v4.2.0](http://api.rubyonrails.org/v4.2.0) doesn't exist. Maybe it's a delay of the documentation deployment?