Update Flags section of README

[ayberk]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
README still states that --cloud-provider flag shouldn't be passed to aws-cloud-controller-manager, which is not the case anymore. This PR updates that section.

Does this PR introduce a user-facing change?:

NONE

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[k8s-ci-robot]

Hi @ayberk. Thanks for your PR.

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[ayberk]

/assign @nckturner

[nckturner]

/lgtm
/approve

[nckturner]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ayberk, nckturner

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