Update KubeOne docs for ARM based installation

[andrealfter]

Installing KubeOne from an ARM based System is NOT mentioned in the docs.

All code examples have amd64 "hard coded". Update would be great.

https://docs.kubermatic.com/kubeone/v1.11/getting-kubeone/

This worked fine for me:

OS=$(uname)
VERSION=$(curl -w '%{url_effective}' -I -L -s -S https://github.com/kubermatic/kubeone/releases/latest -o /dev/null | sed -e 's|.*/v||')

curl -LO "https://github.com/kubermatic/kubeone/releases/download/v${VERSION}/kubeone_${VERSION}_${OS}_arm64.zip"

unzip kubeone_${VERSION}${OS}arm64.zip -d kubeone${VERSION}${OS}arm64
sudo mv kubeone${VERSION}_${OS}_arm64/kubeone /usr/local/bin

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