Remove extra double-dash in error message

[drscotthawley]

Error message should read --main_process_port, not ----main_process_port. Users who copy and paste the message as it was, will get this error message:

Accelerate CLI tool: error: unrecognized arguments: ----main_process_port

What does this PR do?

See above. It fixes a typo in the error message printed when there is a process port mismatch

Fixes # (issue)

N/A

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Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@muellerzr may be interested as this is a CLI issue

[muellerzr]

Thanks for catching this!

[HuggingFaceDocBuilderDev]

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