rust: rbtree: fix minor typo in comment

Fix a typo in a comment to improve clarity and readability.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: #1206
Signed-off-by: Atharv Dubey <atharvd440@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251201165601.31484-1-atharvd440@gmail.com
[ Removed one of the cases that is gone now. Reworded accordingly
  (and to avoid mentioning 'documentation', since it is just
  a comment). - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: DoobyDoPap

rust/kernel/rbtree.rs

@@ -1130,7 +1130,7 @@ pub struct IterMut<'a, K, V> {
 }
 
 // SAFETY: The [`IterMut`] has exclusive access to both `K` and `V`, so it is sufficient to require them to be `Send`.
-// The iterator only gives out immutable references to the keys, but since the iterator has excusive access to those same
+// The iterator only gives out immutable references to the keys, but since the iterator has exclusive access to those same
 // keys, `Send` is sufficient. `Sync` would be okay, but it is more restrictive to the user.
 unsafe impl<'a, K: Send, V: Send> Send for IterMut<'a, K, V> {}