Fix typo on `customize-extractor-error` example #3777

Job	Run time
cargo-hack	4m 51s
check-docs	40s
cargo-public-api-crates (axum)	1m 28s
cargo-public-api-crates (axum-core)	1m 21s
cargo-public-api-crates (axum-extra)	1m 34s
cargo-public-api-crates (axum-macros)	1m 18s
check	1m 36s
cargo-deny check (advisories)	1m 34s
cargo-deny check (bans licenses sources)	2m 2s
dependencies-are-sorted	22s
test-msrv	7m 50s
test-nightly	2m 47s
armv5te-unknown-linux-musleabi	1m 14s
test-docs	2m 50s
wasm32-unknown-unknown	49s
test-versions (stable)	3m 49s
test-versions (beta)	3m 0s
	39m 5s

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