attributes: update async_instrument error text for Rust 1.66

The error message suggesting that you remove a semicolon to return a
value (instead of unit) was updated slightly in rust-lang/rust#102650,
which was included in Rust 1.66.0.

This causes one of the tests in tracing-attributes to fail. We fix it by
using the updated error message.

Author: hds

tracing-attributes/tests/ui/async_instrument.stderr

@@ -93,6 +93,6 @@ error[E0308]: mismatched types
 42 |   async fn extra_semicolon() -> i32 {
    |  ___________________________________^
 43 | |     1;
-   | |      - help: remove this semicolon
+   | |      - help: remove this semicolon to return this value
 44 | | }
    | |_^ expected `i32`, found `()`