docs: fix references for DMA*.txt files

As we moved those files to core-api, fix references to point
to their newer locations.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/37b2fd159fbc7655dbf33b3eb1215396a25f6344.1592895969.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/bus-virt-phys-mapping.txt b/Documentation/bus-virt-phys-mapping.txt
index 4bb07c2..c7bc99c 100644
--- a/Documentation/bus-virt-phys-mapping.txt
+++ b/Documentation/bus-virt-phys-mapping.txt
@@ -8,7 +8,7 @@
 
 	The virt_to_bus() and bus_to_virt() functions have been
 	superseded by the functionality provided by the PCI DMA interface
-	(see Documentation/DMA-API-HOWTO.txt).  They continue
+	(see :doc:`/core-api/dma-api-howto`).  They continue
 	to be documented below for historical purposes, but new code
 	must not use them. --davidm 00/12/12