release-notes: use stderr for header output.

This makes the use of `brew release-notes | pbcopy` a bit easier.

Library/Homebrew/dev-cmd/release-notes.rb

@@ -49,7 +49,7 @@ module Homebrew
       end
     end
 
-    puts "Release notes between #{previous_tag} and #{end_ref}:"
+    $stderr.puts "Release notes between #{previous_tag} and #{end_ref}:"
     puts output
   end
 end