rust: pin-init: fix broken rust doc link

rust: pin-init: fix broken rust doc link

Rust 1.92.0 warns when building the documentation that [`PinnedDrop`] is
an invalid reference. This is correct and it's weird that it didn't warn
before, so fix the link.

[ The reason is that it is hidden -- I had asked about that in the
  upstream PR that changed the behavior because I wasn't sure it was
  intentional (and thus whether we needed to fix this and other cases):

      https://github.com/rust-lang/rust/pull/147153#issuecomment-3395484636

  It turns out it was not, and it has been fixed for 1.92.0's upcoming
  release thanks to Guillaume and León. So we do not strictly need
  this patch and the other changes anymore:

      https://github.com/rust-lang/rust/pull/147809

  However, checking hidden/private items or, even better, a runtime
  toggle to be able to see those on the fly, is something that I think
  would be quite nice so I have had it in our usual lists for a while.
  Guillaume is open to the idea and perhaps experimenting with an
  implementation on our side first -- he asked me to open issues
  upstream:

      https://github.com/rust-lang/rust/issues/149105
      https://github.com/rust-lang/rust/issues/149106

    - Miguel ]

Signed-off-by: Benno Lossin <lossin@kernel.org>
Link: https://patch.msgid.link/20251016211740.653599-1-lossin@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>