docs: update channel toolchain syntax #1733

Job	Run time
build-linux-master	0s
build-freebsd-stable	0s
build-freebsd-master	0s
build-all-features (ubuntu-latest, x86_64-unknown-linux-gnu)	5m 16s
build-all-features (windows-latest, x86_64-pc-windows-msvc)	10m 43s
build-linux-pr (dev, x86_64-unknown-linux-gnu)	5m 20s
build-linux-stable	0s
build-linux-pr (dev, armv7-unknown-linux-gnueabihf)	1m 12s
build-macos-aarch64 (dev, aarch64-apple-darwin)	9m 52s
build-linux-pr (dev, aarch64-linux-android)	1m 11s
build-macos-x86_64 (dev, x86_64-apple-darwin)	8m 57s
build-macos-aarch64 (release, aarch64-apple-darwin)	1m 51s
build-linux-pr (release, x86_64-unknown-linux-gnu)	2m 27s
build-windows-master	0s
build-macos-x86_64 (release, x86_64-apple-darwin)	3m 0s
build-linux-pr (release, armv7-unknown-linux-gnueabihf)	2m 10s
build-windows-pr (dev, x86_64-pc-windows-msvc)	7m 52s
check	36s
build-windows-pr (dev, aarch64-pc-windows-msvc)	2m 20s
build-windows-stable	0s
build-windows-pr (dev, x86_64-pc-windows-gnu)	3m 46s
doc	7s
build-windows-pr (release, x86_64-pc-windows-msvc)	5m 33s
build-windows-pr (release, aarch64-pc-windows-msvc)	5m 16s
build-windows-pr (release, x86_64-pc-windows-gnu)	4m 25s
build-linux-pr (release, aarch64-linux-android)	2m 33s
conclusion	1s
	1h 24m 28s

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