docs: rbtree.txt: fix Sphinx build warnings

Ths file is already at ReST format. Yet, some recent changes
made it to produce a few warnings when building it with
Sphinx.

Those are trivially fixed by marking some literal blocks.

Fix them before adding it to the docs building system.

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