Fix doctest #49

Job	Run time
Fmt check	9s
Test on macos with Rust stable (feat: default)	26s
Test on macos with Rust stable (feat: with-tokio)	35s
Test on macos with Rust 1.68.0 (feat: default)	6m 0s
Test on macos with Rust 1.68.0 (feat: with-tokio)	6m 8s
Test on ubuntu with Rust stable (feat: default)	14s
Test on ubuntu with Rust stable (feat: with-tokio)	31s
Test on ubuntu with Rust 1.68.0 (feat: default)	3m 38s
Test on ubuntu with Rust 1.68.0 (feat: with-tokio)	3m 48s
Test on windows with Rust stable (feat: default)	2m 23s
Test on windows with Rust stable (feat: with-tokio)	2m 46s
Test on windows with Rust 1.68.0 (feat: default)	8m 2s
Test on windows with Rust 1.68.0 (feat: with-tokio)	5m 14s
	39m 54s

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