Documentation: tty: move moxa-smartio.rst to tty

It is a tty_driver, not serial -- uart_driver. So move it to the tty
docs dir too. (The same as the driver itself.)
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220411110143.10019-5-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/serial/index.rst

@@ -16,7 +16,6 @@ Serial drivers
 .. toctree::
     :maxdepth: 1
 
-    moxa-smartio
     serial-iso7816
     serial-rs485
 

Documentation/driver-api/tty/index.rst

@@ -68,5 +68,6 @@ Miscellaneous documentation can be further found in these documents:
 .. toctree::
    :maxdepth: 2
 
+   moxa-smartio
    n_gsm
    n_tty