attributes: update async_instrument error text for Rust 1.66 (#2427)

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