docs: driver-api: generic-counter: fix file path to ABI doc

Fixes: 09e7d4ed

 ("docs: Add Generic Counter interface documentation")
Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>