lib: bitmap.c: get rid of some doc warnings

lib: bitmap.c: get rid of some doc warnings

There are two ascii art drawings there. Use a block markup tag there
in order to get rid of those warnings:

	./lib/bitmap.c:189: WARNING: Unexpected indentation.
	./lib/bitmap.c:190: WARNING: Block quote ends without a blank line; unexpected unindent.
	./lib/bitmap.c:190: WARNING: Unexpected indentation.
	./lib/bitmap.c:191: WARNING: Line block ends without a blank line.

It should be noticed that there's actually a syntax violation
right now, as something like:

	/**
	 ...
	 @src:

will be handled as a definition for @src parameter, and not as
part of a diagram. So, we need to add something before it, in
order for this to be processed the way it should.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1e2568fdfa838c1a0d8cc2a1d70dd4b6de99bfb1.1586881715.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>