docs: correct JoinHandle link in the doc of spawn() (#265) #645

Job	Run time
test (x86_64-unknown-linux-gnu, stable)	4m 28s
test (x86_64-unknown-linux-gnu, nightly)	4m 5s
test (i686-unknown-linux-gnu, stable)	1m 29s
test (i686-unknown-linux-gnu, nightly)	1m 25s
test (aarch64-unknown-linux-gnu, stable)	1m 30s
test (aarch64-unknown-linux-gnu, nightly)	1m 33s
test (armv7-unknown-linux-gnueabihf, stable)	1m 32s
test (armv7-unknown-linux-gnueabihf, nightly)	1m 28s
test (riscv64gc-unknown-linux-gnu, stable)	1m 21s
test (riscv64gc-unknown-linux-gnu, nightly)	1m 27s
test (s390x-unknown-linux-gnu, stable)	1m 28s
test (s390x-unknown-linux-gnu, nightly)	1m 32s
test (x86_64-apple-darwin, stable)	1m 0s
test (x86_64-apple-darwin, nightly)	1m 12s
test (aarch64-apple-darwin, stable)	2m 34s
test (aarch64-apple-darwin, nightly)	3m 1s
test (x86_64-pc-windows-gnu, stable)	4m 18s
test (x86_64-pc-windows-gnu, nightly)	2m 47s
test (x86_64-pc-windows-msvc, stable)	4m 19s
test (x86_64-pc-windows-msvc, nightly)	4m 40s
test (i686-pc-windows-gnu, stable)	4m 54s
test (i686-pc-windows-gnu, nightly)	1m 53s
test (i686-pc-windows-msvc, stable)	3m 57s
test (i686-pc-windows-msvc, nightly)	2m 10s
	1h 0m 3s

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