tty: n_gsm: update doc example to use header for N_GSM0710 define

There is no reason to gues the line discipline number when it is
available from tty.h
Signed-off-by: Martin Hundebøll <martin@geanix.com>
Link: https://lore.kernel.org/r/20190710192656.60381-2-martin@geanix.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/serial/n_gsm.rst

@@ -23,7 +23,7 @@ Major parts of the initialization program :
 (a good starting point is util-linux-ng/sys-utils/ldattach.c)::
 
   #include <linux/gsmmux.h>
-  #define N_GSM0710	21	/* GSM 0710 Mux */
+  #include <linux/tty.h>
   #define DEFAULT_SPEED	B115200
   #define SERIAL_PORT	/dev/ttyS0