Make AM readme example more consistent

At 4a6eba32 mail sending example was rewrited from rails 2 format to
modern rails 3 approach. But passing parameter `recipient` to method
Notifier#welcome was lost.

Passing email as parameter makes this example more solid.

actionmailer/README.rdoc

@@ -67,12 +67,12 @@ simply call the method and optionally call +deliver+ on the return value.
 
 Calling the method returns a Mail Message object:
 
-  message = Notifier.welcome  # => Returns a Mail::Message object
-  message.deliver             # => delivers the email
+  message = Notifier.welcome("david@loudthinking.com")  # => Returns a Mail::Message object
+  message.deliver                                       # => delivers the email
 
 Or you can just chain the methods together like:
 
-  Notifier.welcome.deliver    # Creates the email and sends it immediately
+  Notifier.welcome("david@loudthinking.com").deliver    # Creates the email and sends it immediately
 
 == Setting defaults