Fix a trivial spelling error in the `rust/kernel/str.rs` file.

Fixes: 247b365d

 ("rust: add `kernel` crate")
Reported-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/978


Signed-off-by: Patrick Blass <patrickblass@mailbox.org>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
[Reworded slightly]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>