Update readme insecure flags

[lucid-at-dream]

Fixes # in case of a bug fix, this should point to a bug and any other related issue(s)

Description

Clarifies the usage of --insecure-registry and --skip-tls-verify-registry command line
arguments. They were being referred to as flags, which mislead my into believing they
did not require a value. This PR updates the ReadMe to be a bit more clear.

Submitter Checklist

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• Includes unit tests
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Reviewer Notes

• The code flow looks good.
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Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:
- kaniko adds a new flag `--registry-repo` to override registry

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[lucid-at-dream]

@googlebot I signed it!