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: ad2907b ("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>

Author: ojeda

rust/kernel/dma.rs

@@ -94,7 +94,7 @@ pub mod attrs {
     pub const DMA_ATTR_ALLOC_SINGLE_PAGES: Attrs = Attrs(bindings::DMA_ATTR_ALLOC_SINGLE_PAGES);
 
     /// This tells the DMA-mapping subsystem to suppress allocation failure reports (similarly to
-    /// __GFP_NOWARN).
+    /// `__GFP_NOWARN`).
     pub const DMA_ATTR_NO_WARN: Attrs = Attrs(bindings::DMA_ATTR_NO_WARN);
 
     /// Used to indicate that the buffer is fully accessible at an elevated privilege level (and