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>