rust: rbtree: fix minor typo in comment

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: https://github.com/Rust-for-Linux/linux/issues/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>