docs: serial: fix a reference file name in driver.rst

Fix the following 'make refcheckdocs' warning:
Warning: Documentation/driver-api/serial/driver.rst references a file
that doesn't exist: Documentation/driver-api/serial/tty.rst

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20220304100315.6732-1-wanjiabing@vivo.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: Wan Jiabing

Documentation/driver-api/serial/driver.rst

@@ -311,7 +311,7 @@ hardware.
 	This call must not sleep
 
   set_ldisc(port,termios)
-	Notifier for discipline change. See Documentation/driver-api/serial/tty.rst.
+	Notifier for discipline change. See Documentation/tty/tty_ldisc.rst.
 
 	Locking: caller holds tty_port->mutex