rust: str: fix typo in comment

Fix a typo ("then" to "than") in a comment.

Signed-off-by: Jihed Chaibi <jihed.chaibi.dev@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Fixes: fffed679 ("rust: str: add `Formatter` type")
Link: https://lore.kernel.org/r/20250517002604.603223-1-jihed.chaibi.dev@gmail.com


[ Reworded. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>