Update Action Mailer guide on how to add images

Unlike controllers, mailers can't infer the protocol from the request,
so users need to specify it.

guides/source/action_mailer_basics.md

@@ -634,9 +634,13 @@ As the `:asset_host` usually is consistent across the application you can
 configure it globally in `config/application.rb`:
 
 ```ruby
-config.asset_host = 'http://example.com'
+config.action_mailer.asset_host = 'http://example.com'
 ```
 
+NOTE: Because we can't infer the protocol from the request, you'll need to
+specify a protocol such as `http://` or `https://` in the
+`:asset_host` config.
+
 Now you can display an image inside your email.
 
 ```html+erb