fix link to payload_as_str (#1402)

Author: RalfJung

posts/2024-09-05-Rust-1.81.0.md

@@ -128,7 +128,7 @@ context (where panics can have an arbitrary payload), while
 [`#[panic_handler]`](https://doc.rust-lang.org/nomicon/panic-handler.html) in
 `#![no_std]` context (where panics always carry a formatted *message*). Separating
 these types allows us to add more useful methods to these types, such as
-[`std::panic::PanicHookInfo::payload_as_str()`]() and
+[`std::panic::PanicHookInfo::payload_as_str()`](https://doc.rust-lang.org/stable/std/panic/struct.PanicHookInfo.html#method.payload_as_str) and
 [`core::panic::PanicInfo::message()`](https://doc.rust-lang.org/stable/core/panic/struct.PanicInfo.html#method.message).
 
 [`std::panic::PanicInfo`]: https://doc.rust-lang.org/stable/std/panic/type.PanicInfo.html