fix some doc test error #669

Job	Run time
test (x86_64-unknown-linux-gnu, stable)	4m 31s
test (x86_64-unknown-linux-gnu, nightly)	5m 2s
test (aarch64-unknown-linux-gnu, stable)	1m 49s
test (aarch64-unknown-linux-gnu, nightly)	5m 9s
test (armv7-unknown-linux-gnueabihf, stable)	1m 39s
test (armv7-unknown-linux-gnueabihf, nightly)	2m 58s
test (riscv64gc-unknown-linux-gnu, stable)	2m 3s
test (riscv64gc-unknown-linux-gnu, nightly)	2m 59s
test (s390x-unknown-linux-gnu, stable)	1m 44s
test (s390x-unknown-linux-gnu, nightly)	3m 9s
test (x86_64-apple-darwin, stable)	1m 28s
test (x86_64-apple-darwin, nightly)	4m 18s
test (aarch64-apple-darwin, stable)	2m 33s
test (aarch64-apple-darwin, nightly)	3m 18s
test (x86_64-pc-windows-gnu, stable)	3m 14s
test (x86_64-pc-windows-gnu, nightly)	5m 33s
test (x86_64-pc-windows-msvc, stable)	4m 26s
test (x86_64-pc-windows-msvc, nightly)	4m 55s
test (i686-pc-windows-gnu, stable)	5m 15s
test (i686-pc-windows-gnu, nightly)	9m 0s
test (i686-pc-windows-msvc, stable)	2m 29s
test (i686-pc-windows-msvc, nightly)	3m 26s
	1h 20m 58s

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