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

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

Since b9e851cd4a87 ("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.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>