Documentation: input: fix path to input code defnitions

The UAPI header split failed to update the documentation in
input-programming.txt; fix things accordingly.
Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/input/input-programming.txt

@@ -174,8 +174,8 @@ It's reported to the input system via:
 
 	input_report_key(struct input_dev *dev, int code, int value)
 
-See linux/input.h for the allowable values of code (from 0 to KEY_MAX).
-Value is interpreted as a truth value, ie any nonzero value means key
+See uapi/linux/input-event-codes.h for the allowable values of code (from 0 to
+KEY_MAX). Value is interpreted as a truth value, ie any nonzero value means key
 pressed, zero value means key released. The input code generates events only
 in case the value is different from before.