Documentation: fix input related doc refs

Make `input` document refs valid including:
  - joystick
  - joystick-parport

Signed-off-by: Tom Saeger <tom.saeger@oracle.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/hid/hiddev.txt

@@ -12,7 +12,7 @@ To support these disparate requirements, the Linux USB system provides
 HID events to two separate interfaces:
 * the input subsystem, which converts HID events into normal input
 device interfaces (such as keyboard, mouse and joystick) and a
-normalised event interface - see Documentation/input/input.txt
+normalised event interface - see Documentation/input/input.rst
 * the hiddev interface, which provides fairly raw HID events
 
 The data flow for a HID event produced by a device is something like