Documentation: kgdb: Replace deprecated remotebaud

Using set remotebaud to set the baud rate was deprecated in
gdb-7.7 and completely removed from the command parser in gdb-7.8
(released in 2014). Adopt set serial baud instead.

Signed-off-by: Christian Loehle <cloehle@hyperstone.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/4050689967ed46baaa3bfadda53a0e73@hyperstone.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>