rust: pin-init: Update the structural pinning link in readme.

The previous link anchor was broken in rust 1.77, because the
documentation was refactored in upstream rust.
Change the link to refer to the new section in the rust documentation.

Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Link: https://github.com/Rust-for-Linux/pin-init/pull/37/commits/a146142fe18cafa52f8c6da306ca2729d789cfbf


[ Fixed commit authorship. - Benno ]
Signed-off-by: Benno Lossin <benno.lossin@proton.me>