docs: update channel toolchain syntax #1727

Job	Run time
build-linux-master	0s
build-all-features (ubuntu-latest, x86_64-unknown-linux-gnu)	4m 51s
build-freebsd-stable	0s
build-freebsd-master	0s
build-all-features (windows-latest, x86_64-pc-windows-msvc)	10m 1s
build-linux-pr (dev, x86_64-unknown-linux-gnu)	5m 32s
build-linux-stable	0s
build-linux-pr (dev, armv7-unknown-linux-gnueabihf)	1m 31s
build-macos-x86_64 (dev, x86_64-apple-darwin)	9m 19s
build-macos-aarch64 (dev, aarch64-apple-darwin)	8m 19s
build-linux-pr (dev, aarch64-linux-android)	1m 16s
build-macos-x86_64 (release, x86_64-apple-darwin)	3m 29s
build-macos-aarch64 (release, aarch64-apple-darwin)	1m 46s
build-linux-pr (release, x86_64-unknown-linux-gnu)	2m 34s
build-windows-master	0s
build-linux-pr (release, armv7-unknown-linux-gnueabihf)	2m 19s
build-linux-pr (release, aarch64-linux-android)	2m 20s
build-windows-pr (dev, x86_64-pc-windows-msvc)	9m 10s
build-windows-stable	0s
build-windows-pr (dev, aarch64-pc-windows-msvc)	4m 36s
check	44s
build-windows-pr (dev, x86_64-pc-windows-gnu)	4m 13s
doc	8s
build-windows-pr (release, x86_64-pc-windows-msvc)	5m 57s
build-windows-pr (release, aarch64-pc-windows-msvc)	4m 0s
build-windows-pr (release, x86_64-pc-windows-gnu)	7m 39s
conclusion	1s
	1h 29m 45s

Provide feedback