Update MetalLB README.md (#6350)
Update MetalLB README.md (#6350)

Recently MetalLB becomes one of addons with renaming the options.
This updates MetalLB README.md for this change.
kubernetes-sigs/kubespray,Atoms/kubespray,kubernetes-incubator/kargo,kubernetes-sigs/kubespray,kubernetes-incubator/kubespray,kubernetes-sigs/kubespray,kubernetes-incubator/kargo,kubernetes-sigs/kubespray,kubernetes-incubator/kubespray,Atoms/kubespray,Atoms/kubespray,Atoms/kubespray