media: hist-v4l2.rst: remove struct duplication

There are several places there where struct was used twice,
on multiple lines, like:

	struct
	struct ``v4l2_zoomcap``

That's probably done during the DocBook to ReSt conversion.

Probably the cases where "struct struct" were at the same line
was addressed back then, but the multi-line cases are still
there.

Get rid of them.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>