net: update AsRawFd doc link to current Rust stdlib location (#7429)

Author: GarmashAlex

tokio/src/net/tcp/socket.rs

@@ -81,7 +81,7 @@ cfg_net! {
     ///
     /// [`RawFd`]: https://doc.rust-lang.org/std/os/fd/type.RawFd.html
     /// [`RawSocket`]: https://doc.rust-lang.org/std/os/windows/io/type.RawSocket.html
-    /// [`AsRawFd`]: https://doc.rust-lang.org/std/os/unix/io/trait.AsRawFd.html
+    /// [`AsRawFd`]: https://doc.rust-lang.org/std/os/fd/trait.AsRawFd.html
     /// [`AsRawSocket`]: https://doc.rust-lang.org/std/os/windows/io/trait.AsRawSocket.html
     /// [`socket2`]: https://docs.rs/socket2/
     #[cfg_attr(docsrs, doc(alias = "connect_std"))]