Update SSH documentation for ubuntu

[jpugliesi]

We've found that, with kops 1.19, the bastion and k8s nodes user is ubuntu when using the default kops configuration

[k8s-ci-robot]

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[bharath-123]

/ok-to-test

[bharath-123]

/lgtm

[hakman]

Thanks @jpugliesi.
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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