Add missing Markdown code span.

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

Fixes: ad2907b4e308 ("rust: add dma coherent allocator abstraction")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/dma.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
index 8cdc76043ee7..35fd8a638473 100644
--- a/rust/kernel/dma.rs
+++ b/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
-- 
2.49.0