docs: block: blk-mq.rst: get rid of :c:type

The :c:type macros are not used properly there, as reported
by Sphinx 3:

	./Documentation/block/blk-mq.rst:112: WARNING: Unparseable C cross-reference: 'hctx->dispatch'
	Invalid C declaration: Expected end of definition. [error at 4]
	  hctx->dispatch
	  ----^

Also, they won't be generating any cross references.

So, replace them by a literal markup.
Reviewed-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/block/blk-mq.rst

@@ -110,9 +110,9 @@ block layer removes requests from the associated software queues and tries to
 dispatch to the hardware.
 
 If it's not possible to send the requests directly to hardware, they will be
-added to a linked list (:c:type:`hctx->dispatch`) of requests. Then,
+added to a linked list (``hctx->dispatch``) of requests. Then,
 next time the block layer runs a queue, it will send the requests laying at the
-:c:type:`dispatch` list first, to ensure a fairness dispatch with those
+``dispatch`` list first, to ensure a fairness dispatch with those
 requests that were ready to be sent first. The number of hardware queues
 depends on the number of hardware contexts supported by the hardware and its
 device driver, but it will not be more than the number of cores of the system.