rust: kernel: add missing backticks in doc comment

This is part of the effort to minimize the differences of the `rust`
branch with respect to mainline in order to eventually drop it.

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

Author: ojeda

rust/kernel/lib.rs

@@ -117,7 +117,7 @@ use core::marker::PhantomData;
 /// [`PAGE_SHIFT`]: ../../../include/asm-generic/page.h
 pub const PAGE_SIZE: usize = 1 << bindings::PAGE_SHIFT;
 
-/// Prefix to appear before log messages printed from within the kernel crate.
+/// Prefix to appear before log messages printed from within the `kernel` crate.
 const __LOG_PREFIX: &[u8] = b"rust_kernel\0";
 
 /// The top level entrypoint to implementing a kernel module.