doc: clarify the format of --registry-mirror

[zhouhaibing089]

Description

In the context of Docker Hub ratelimit, registry mirror has been applied
more and more. It's very unfortunate that kaniko doesn't take the same
form as docker/containerd does. The registry mirror shouldn't have scheme
like https:// included. This is not explicitly described in the doc.

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Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

None.

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