Fix grammar on AM.mailer_name documentation.

actionmailer/lib/action_mailer/base.rb

@@ -412,7 +412,7 @@ def register_interceptor(interceptor)
         Mail.register_interceptor(delivery_interceptor)
       end
 
-      # Returns the name of current mailer. This method also being used as a path for a view lookup.
+      # Returns the name of current mailer. This method is also being used as a path for a view lookup.
       # If this is an anonymous mailer, this method will return +anonymous+ instead.
       def mailer_name
         @mailer_name ||= anonymous? ? "anonymous" : name.underscore