Fix clippy warning on `FromRef` derive macro #4632

Job	Run time
cargo-hack	5m 53s
check	41s
check-docs	26s
cargo-public-api-crates (axum)	39s
dependencies-are-sorted	20s
cargo-public-api-crates (axum-core)	20s
cargo-deny check (advisories)	23s
Spell Check with Typos	5s
cargo-public-api-crates (axum-extra)	34s
cargo-deny check (bans licenses sources)	28s
cargo-public-api-crates (axum-macros)	15s
test-msrv	2m 33s
armv5te-unknown-linux-musleabi	50s
test-docs	1m 50s
test-nightly	2m 10s
test-versions (stable)	2m 29s
wasm32-unknown-unknown	21s
test-versions (beta)	1m 18s
	21m 35s

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