Update Cargo.toml readme field to point to workspace file #4188

Job	Run time
check	2m 11s
cargo-public-api-crates (axum)	58s
cargo-hack	5m 49s
cargo-public-api-crates (axum-core)	46s
check-docs	33s
cargo-public-api-crates (axum-extra)	59s
cargo-deny check (advisories)	22s
cargo-public-api-crates (axum-macros)	49s
dependencies-are-sorted	44s
cargo-deny check (bans licenses sources)	23s
Spell Check with Typos	6s
test-nightly	2m 17s
wasm32-unknown-unknown	30s
test-versions (stable)	4m 19s
test-msrv	4m 31s
test-docs	2m 8s
test-versions (beta)	4m 28s
armv5te-unknown-linux-musleabi	1m 1s
	32m 54s

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