rust: block: fix comment referring to states by wrong labels

The comment in `Request` describing the four ownership states referred
to states 2 and 3 as "B" and "C", which are inconsistent with the
numbering scheme used throughout the same comment block.

Update the labels to use "2" and "3" to match the existing numbered list.

Fixes: a307bf1 ("rust: block: convert `block::mq` to use `Refcount`")
Signed-off-by: Hsiu Che Yu <[email protected]>

Author: HsiuCheYu

rust/kernel/block/mq/request.rs

@@ -39,7 +39,7 @@ use core::{marker::PhantomData, ptr::NonNull};
 /// back ownership to the block layer.
 ///
 /// Note that the driver can still obtain new `ARef` even if there is no `ARef`s in existence by
-/// using `tag_to_rq`, hence the need to distinguish B and C.
+/// using `tag_to_rq`, hence the need to distinguish 2 and 3.
 ///
 /// The states are tracked through the private `refcount` field of
 /// `RequestDataWrapper`. This structure lives in the private data area of the C