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>

Documentation/DocBook/device-drivers.tmpl

@@ -217,8 +217,8 @@ X!Isound/sound_firmware.c
   <chapter id="uart16x50">
      <title>16x50 UART Driver</title>
 !Iinclude/linux/serial_core.h
-!Edrivers/serial/serial_core.c
-!Edrivers/serial/8250.c
+!Edrivers/tty/serial/serial_core.c
+!Edrivers/tty/serial/8250.c
   </chapter>
 
   <chapter id="fbdev">