Input: joystick - refer to /dev/input/js0 in documentation

Nowadays the joystick device nodes are created under /dev/input, reflect
this in the documentation in order to make copy and paste easier for
users.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>