rust: condvar: fix broken intra-doc link

The future move of pin-init to `syn` uncovers the following broken
intra-doc link:

    error: unresolved link to `crate::pin_init`
      --> rust/kernel/sync/condvar.rs:39:40
       |
    39 | /// instances is with the [`pin_init`](crate::pin_init!) and [`new_condvar`] macros.
       |                                        ^^^^^^^^^^^^^^^^ no item named `pin_init` in module `kernel`
       |
       = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
       = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`

Currently, when rendered, the link points to a literal `crate::pin_init!`
URL.

Thus fix it.

Cc: stable@vger.kernel.org
Fixes: 129e97be ("rust: pin-init: fix documentation links")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251029073344.349341-1-ojeda@kernel.org


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