Documentation: kgdb: Correct parameter error

Module kgdb had been converted to debug_core since commit c4338209
("Move kernel/kgdb.c to kernel/debug/debug_core.c") be added, so let's
correct the module parameter path.

Fixes: c4338209 ("Move kernel/kgdb.c to kernel/debug/debug_core.c")
Signed-off-by: Changhuang Liang <changhuang.liang@starfivetech.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240925100742.1730576-1-changhuang.liang@starfivetech.com