Documentation: core-api: update kernel-doc reference to kmod.c

Documentation: core-api: update kernel-doc reference to kmod.c

Commit d6f819908f8aac ("module: fold usermode helper kmod into modules
directory") moves kmod helper implementation (kmod.c) to kernel/module/
directory but forgets to update its reference on kernel api doc, hence:

WARNING: kernel-doc './scripts/kernel-doc -rst -enable-lineno -sphinx-version 2.4.4 -export ./kernel/kmod.c' failed with return code 2

Update the reference.

Fixes: d6f819908f8aac ("module: fold usermode helper kmod into modules directory")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/linux-next/20230324154413.19cc78be@canb.auug.org.au/
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Liam Beguin <liambeguin@gmail.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>