fix ActionMailer readme example

actionmailer/README.rdoc

@@ -59,6 +59,8 @@ generated would look like this:
 
   Mr. david@loudthinking.com
 
+  Thank you for signing up!
+
 In previous version of Rails you would call <tt>create_method_name</tt> and
 <tt>deliver_method_name</tt>.  Rails 3.0 has a much simpler interface, you
 simply call the method and optionally call +deliver+ on the return value.