Mention tokio-stream in SSE example #3821

Job	Run time
check-docs	39s
cargo-hack	7m 0s
cargo-public-api-crates (axum)	1m 23s
cargo-public-api-crates (axum-core)	1m 6s
cargo-public-api-crates (axum-extra)	1m 20s
cargo-public-api-crates (axum-macros)	1m 11s
check	3m 20s
dependencies-are-sorted	1m 31s
cargo-deny check (advisories)	30s
cargo-deny check (bans licenses sources)	33s
test-versions (stable)	3m 53s
test-versions (beta)	6m 42s
test-docs	4m 52s
test-msrv	6m 54s
armv5te-unknown-linux-musleabi	1m 20s
test-nightly	4m 4s
wasm32-unknown-unknown	33s
	46m 51s

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