docs: block: biodoc.rst: avoid using ReST :doc:`foo` markup

The :doc:`foo` tag is auto-generated via automarkup.py.
So, use the filename at the sources, instead of :doc:`foo`.
Signed-off-by: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1d26256b305e02da82a6a990910a5b5fb9a0355e.1623824363.git.mchehab+huawei@kernel.orgSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/block/biodoc.rst

@@ -196,7 +196,7 @@ a virtual address mapping (unlike the earlier scheme of virtual address
 do not have a corresponding kernel virtual address space mapping) and
 low-memory pages.
 
-Note: Please refer to :doc:`/core-api/dma-api-howto` for a discussion
+Note: Please refer to Documentation/core-api/dma-api-howto.rst for a discussion
 on PCI high mem DMA aspects and mapping of scatter gather lists, and support
 for 64 bit PCI.