rust: str: fix requierments->requirements typo

rust: str: fix requierments->requirements typo

[ Upstream commit 88e8c2ec4ab84f9f05ed5af9693a3972baf386c4 ]

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

Fixes: 247b365dc8dc ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>