rust: str: fix requierments->requirements typo

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

Fixes: 247b365 ("rust: add `kernel` crate")
Reported-by: Miguel Ojeda <[email protected]>
Link: #978
Signed-off-by: Patrick Blass <[email protected]>
Reviewed-by: Vincenzo Palazzo <[email protected]>
[Reworded slightly]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: Patrick Blass

Diff for: rust/kernel/str.rs

@@ -408,7 +408,7 @@ impl RawFormatter {
     /// If `pos` is less than `end`, then the region between `pos` (inclusive) and `end`
     /// (exclusive) must be valid for writes for the lifetime of the returned [`RawFormatter`].
     pub(crate) unsafe fn from_ptrs(pos: *mut u8, end: *mut u8) -> Self {
-        // INVARIANT: The safety requierments guarantee the type invariants.
+        // INVARIANT: The safety requirements guarantee the type invariants.
         Self {
             beg: pos as _,
             pos: pos as _,