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>
diff --git a/drivers/ata/sata_sx4.c b/drivers/ata/sata_sx4.c
index 48301cb..405e606 100644
--- a/drivers/ata/sata_sx4.c
+++ b/drivers/ata/sata_sx4.c
@@ -24,7 +24,7 @@
  *
  *
  *  libata documentation is available via 'make {ps|pdf}docs',
- *  as Documentation/DocBook/libata.*
+ *  as Documentation/driver-api/libata.rst
  *
  *  Hardware documentation available under NDA.
  *