Documentation: dma-buf: heaps: Fix code markup

Documentation: dma-buf: heaps: Fix code markup

Code snippets should be wrapped in double backticks to follow
reStructuredText semantics; the use of single backticks uses the
:title-reference: role by default, which isn't quite what we want.
Add double backticks to code snippets to fix this.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Jared Kangas <jkangas@redhat.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://lore.kernel.org/r/20250610131231.1724627-2-jkangas@redhat.com