counter: fix docum. build problems after filename change

Fix documentation build warnings due to a source file being
renamed.

WARNING: kernel-doc '../scripts/kernel-doc -rst -enable-lineno -sphinx-version 1.8.5 -export ../drivers/counter/counter.c' failed with return code 2

Error: Cannot open file ../drivers/counter/counter.c

Fixes: aaec1a0f ("counter: Internalize sysfs interface code")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: William Breathitt Gray <vilhelm.gray@gmail.com>
Cc: linux-iio@vger.kernel.org
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Acked-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Link: https://lore.kernel.org/r/20211005055157.22937-1-rdunlap@infradead.orgSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/driver-api/generic-counter.rst

@@ -234,7 +234,7 @@ for defining a counter device.
 .. kernel-doc:: include/linux/counter.h
    :internal:
 
-.. kernel-doc:: drivers/counter/counter.c
+.. kernel-doc:: drivers/counter/counter-core.c
    :export:
 
 Driver Implementation