docbook: fix 8250 naming in device-drivers

8250 driver has been (re)renamed to 8250_core.c by commit
9196d8ac. Follow that change to fix the
following error when building htmldocs:

  docproc: /work/cross/linux//drivers/tty/serial/8250/8250.c: No such file or directory
Acked-by: Rob landley <rob@landley.net>
Signed-off-by: Vincent Stehlé <v-stehle@ti.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/DocBook/device-drivers.tmpl

@@ -227,7 +227,7 @@ X!Isound/sound_firmware.c
   <chapter id="uart16x50">
      <title>16x50 UART Driver</title>
 !Edrivers/tty/serial/serial_core.c
-!Edrivers/tty/serial/8250/8250.c
+!Edrivers/tty/serial/8250/8250_core.c
   </chapter>
 
   <chapter id="fbdev">