Use correct domain

Author: OskarStark

mailer.rst

@@ -173,8 +173,8 @@ party provider:
     .. code-block:: env
 
         # .env
-        MAILER_DSN=mailgun+https://KEY:DOMAIN@example.com
-        MAILER_DSN=mailgun+https://KEY:DOMAIN@example.com:99
+        MAILER_DSN=mailgun+https://KEY:DOMAIN@requestbin.com
+        MAILER_DSN=mailgun+https://KEY:DOMAIN@requestbin.com:99
 
     Note that the protocol is *always* HTTPs and cannot be changed.