Fix missing backtick

Author: PrOF-kk

mailer.rst

@@ -276,7 +276,7 @@ party provider:
 .. note::
 
     The specific transports, e.g. ``mailgun+smtp`` are designed to work without any manual configuration.
-    Changing the port by appending it to your DSN is not supported for any of these ``<provider>+smtp` transports.
+    Changing the port by appending it to your DSN is not supported for any of these ``<provider>+smtp`` transports.
     If you need to change the port, use the ``smtp`` transport instead, like so:
 
     .. code-block:: env