Documentation: tty: move n_gsm to tty

We have n_tty documented in Documentation/driver-api/tty/. n_gsm belongs
there too, so move from serial/ to tty/ too.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220411110143.10019-4-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/serial/index.rst

@@ -17,7 +17,6 @@ Serial drivers
     :maxdepth: 1
 
     moxa-smartio
-    n_gsm
     serial-iso7816
     serial-rs485
 

Documentation/driver-api/tty/index.rst

@@ -68,4 +68,5 @@ Miscellaneous documentation can be further found in these documents:
 .. toctree::
    :maxdepth: 2
 
+   n_gsm
    n_tty