ata: update references for libata documentation

The libata documentation is now using ReST. Update references
to it to point to the new place.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/drivers/ata/ahci.h b/drivers/ata/ahci.h
index 5db6ab2..30f67a1 100644
--- a/drivers/ata/ahci.h
+++ b/drivers/ata/ahci.h
@@ -24,7 +24,7 @@
  *
  *
  * libata documentation is available via 'make {ps|pdf}docs',
- * as Documentation/DocBook/libata.*
+ * as Documentation/driver-api/libata.rst
  *
  * AHCI hardware documentation:
  * http://www.intel.com/technology/serialata/pdf/rev1_0.pdf