docbook: fix broken serial to tty/serial movement

Fix move of drivers/serial/ to drivers/tty/, where it broke
one of the docbook files:

  docproc: drivers/serial/serial_core.c: No such file or directory

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>