docs/driver-api: Replace a comma in the n_gsm.rst with a double colon

Since b9e851cd ("tty: n_gsm: Add some instructions and code for requester") which
introduced a warning:

linux/Documentation/driver-api/serial/n_gsm.rst:23: WARNING: Definition list ends without a blank line; unexpected unindent.
linux/Documentation/driver-api/serial/n_gsm.rst:100: WARNING: Definition list ends without a blank line; unexpected unindent.
linux/Documentation/driver-api/serial/n_gsm.rst:115: WARNING: Inline emphasis start-string without end-string.
linux/Documentation/driver-api/serial/n_gsm.rst:118: WARNING: Inline emphasis start-string without end-string.
linux/Documentation/driver-api/serial/n_gsm.rst:120: WARNING: Inline emphasis start-string without end-string.
linux/Documentation/driver-api/serial/n_gsm.rst:122: WARNING: Inline emphasis start-string without end-string.
linux/Documentation/driver-api/serial/n_gsm.rst:125: WARNING: Inline emphasis start-string without end-string.
linux/Documentation/driver-api/serial/n_gsm.rst:139: WARNING: Inline emphasis start-string without end-string.

A paragraph consisting of two colons ("::") signifies that the following text block(s) comprise a literal block.
Add soome blank lines.
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/20211209092439.562433-1-siyanteng@loongson.cnSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/serial/n_gsm.rst

@@ -18,9 +18,12 @@ How to use it
 1.1 initialize the modem in 0710 mux mode (usually AT+CMUX= command) through
     its serial port. Depending on the modem used, you can pass more or less
     parameters to this command.
+
 1.2 switch the serial line to using the n_gsm line discipline by using
     TIOCSETD ioctl.
+
 1.3 configure the mux using GSMIOC_GETCONF / GSMIOC_SETCONF ioctl.
+
 1.4 obtain base gsmtty number for the used serial port.
 
 Major parts of the initialization program :
@@ -95,10 +98,13 @@ Major parts of the initialization program :
 
 2.1 receive string "AT+CMUX= command" through its serial port,initialize
     mux mode config
+
 2.2 switch the serial line to using the n_gsm line discipline by using
     TIOCSETD ioctl.
+
 2.3 configure the mux using GSMIOC_GETCONF / GSMIOC_SETCONF ioctl.
-2.4 obtain base gsmtty number for the used serial port,
+
+2.4 obtain base gsmtty number for the used serial port::
 
   #include <stdio.h>
   #include <stdint.h>