Follow-up to #49995.

Prior to this commit, `link:classes/...` URLs were used in `README.rdoc`
files so that the URLs would be versioned when rendered at
api.rubyonrails.org.  However, outside of api.rubyonrails.org, such URLs
aren't properly resolved.

Since rails/sdoc#345, `https://api.rubyonrails.org/classes/...` URLs
will also be versioned when rendered at api.rubyonrails.org, and such
URLs are properly resolved everywhere.  Therefore, this commit converts
`link:classes/...` URLs to that form.