rust: alloc: 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: dd09538fb4 ("rust: alloc: implement `Cmalloc` in module allocator_test")
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250324210359.1199574-5-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/alloc/allocator_test.rs

@@ -4,7 +4,7 @@
 //! of those types (e.g. `CString`) use kernel allocators for instantiation.
 //!
 //! In order to allow userspace test cases to make use of such types as well, implement the
-//! `Cmalloc` allocator within the allocator_test module and type alias all kernel allocators to
+//! `Cmalloc` allocator within the `allocator_test` module and type alias all kernel allocators to
 //! `Cmalloc`. The `Cmalloc` allocator uses libc's `realloc()` function as allocator backend.
 
 #![allow(missing_docs)]