Documentation: serial: fix header path

RS485 related structure will be defined in user space API
header.
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/serial/serial-rs485.txt

@@ -132,5 +132,5 @@
 
 5. REFERENCES
 
- [1]	include/linux/serial.h
+ [1]	include/uapi/linux/serial.h
  [2]	Documentation/devicetree/bindings/serial/rs485.txt