task: fix broken link in AbortHandle RustDoc (#4545)

## Motivation

There's a broken docs link in the docs for `AbortHandle`. Somehow this
managed to slip past CI yesterday when #4530 was merged
(https://github.com/tokio-rs/tokio/runs/5325278596?check_suite_focus=true)
but it's breaking the build now
(https://github.com/tokio-rs/tokio/runs/5337182732?check_suite_focus=true)
which seems really weird to me, but...whatever...

## Solution

This branch fixes the broken link lol.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tokio/src/runtime/task/abort.rs

@@ -38,6 +38,7 @@ impl AbortHandle {
     /// this method will do nothing.
     ///
     /// [cancelled]: method@super::error::JoinError::is_cancelled
+    /// [`JoinHandle::abort`]: method@super::JoinHandle::abort
     // the `AbortHandle` type is only publicly exposed when `tokio_unstable` is
     // enabled, but it is still defined for testing purposes.
     #[cfg_attr(not(tokio_unstable), allow(unreachable_pub))]