rust: pin-init: fix typos

Correct two typos in the `Wrapper::pin_init` documentation.

Link: https://github.com/Rust-for-Linux/pin-init/pull/48/commits/fd0bf5e244b685188dc642fc4a0bd3f042468fdb


Reviewed-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>