Fix rustdoc warning

```
warning: `crate::task::panic_waker` is both a module and a function
  --> futures-test/src/task/mod.rs:26:23
   |
26 | //! - [`panic_waker`](crate::task::panic_waker) creates a waker that panics when
   |                       ^^^^^^^^^^^^^^^^^^^^^^^^ ambiguous link
   |
   = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default
help: to link to the module, prefix with `mod@`
   |
26 | //! - [`panic_waker`](mod@crate::task::panic_waker) creates a waker that panics when
   |                       ++++
help: to link to the function, add parentheses
   |
26 | //! - [`panic_waker`](crate::task::panic_waker()) creates a waker that panics when
   |                                               ++
```

Author: taiki-e

futures-test/src/task/mod.rs

@@ -23,7 +23,7 @@
 //! Test wakers:
 //! - [`noop_waker`](crate::task::noop_waker) creates a waker that ignores calls to
 //!   [`wake`](futures_core::task::Waker).
-//! - [`panic_waker`](crate::task::panic_waker) creates a waker that panics when
+//! - [`panic_waker`](crate::task::panic_waker()) creates a waker that panics when
 //!   [`wake`](futures_core::task::Waker) is called.
 //! - [`new_count_waker`](crate::task::new_count_waker) creates a waker that increments a counter whenever
 //!   [`wake`](futures_core::task::Waker) is called.