Update README.md

[omotenko]

Wrong cable name.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @senny (or someone else) soon.

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[senny]

Context: 26bcf81

There's another occurrence, that we should change alongside: https://github.com/omotenko/rails/blame/patch-1/actioncable/README.md#L411 (See 8615f8b)

@omotenko thank you.