rust: dma: add missing Markdown code span

Add missing Markdown code span.

This was found using the Clippy `doc_markdown` lint, which we may want
to enable.

Fixes: ad2907b4 ("rust: add dma coherent allocator abstraction")
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250324210359.1199574-6-ojeda@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>