rust: pin-init: fix Rust 1.90.0 doc(hidden) broken intra-doc link

Starting with Rust 1.90.0 (expected 2025-12-11), Rust checks intra-doc
links in `doc(hidden)` items [1]:

    warning: unresolved link to `PinnedDrop`
       --> rust/pin-init/src/macros.rs:508:11
        |
    508 | /// See [`PinnedDrop`] for more information.
        |           ^^^^^^^^^^ no item named `PinnedDrop` in scope
        |
        = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
        = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

Thus fix it. Normally, we would fix the link, but there are other
`PinnedDrop` mentions in the file without the intra-doc link, so stay
consistent.

Link: rust-lang/rust#147153 [1]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

rust/pin-init/src/macros.rs

@@ -505,7 +505,7 @@ pub use ::paste::paste;
 
 /// Creates a `unsafe impl<...> PinnedDrop for $type` block.
 ///
-/// See [`PinnedDrop`] for more information.
+/// See `PinnedDrop` for more information.
 #[doc(hidden)]
 #[macro_export]
 macro_rules! __pinned_drop {