counter: fix Documentation build error due to incorrect source file name

Fix kernel-doc build error in Documentation/driver-api/generic-counter.rst
of incorrect source file name.
Fixes this warning and error:

Error: Cannot open file ../drivers/counter/generic-counter.c
WARNING: kernel-doc '../scripts/kernel-doc -rst -enable-lineno -export ../drivers/counter/generic-counter.c' failed with return code 2

Fixes: 09e7d4ed ("docs: Add Generic Counter interface documentation")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>