The trailing '/' isn't being picked up by Github anyway, and the link works as is.

actionmailer/README.rdoc

@@ -141,7 +141,7 @@ The latest version of Action Mailer can be installed with Rubygems:
 
 Source code can be downloaded as part of the Rails project on GitHub
 
-* https://github.com/rails/rails/tree/master/actionmailer/
+* https://github.com/rails/rails/tree/master/actionmailer
 
 
 == License