Update readme.md, add missing --k3s-channel flag

[Vashiru]

Description

Add missing command line flag for k3s-channel. Description taken from: https://github.com/alexellis/k3sup/blob/fd9dfeaa6cd32f0d048f33705a04c14ca4aa3550/cmd/install.go#L86-L88

Motivation and Context

Missing documentation

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)

[derek[bot]]

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