Add missing word header

[Johennes]

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• Pull request is classified as 'other changes'

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[Johennes]

On a related note, it seems like sending the retry_after_ms response field was downgraded from SHOULD to MAY in 1.10. I think for a proper deprecation it should have stayed SHOULD so that servers include it whenever they set the new header to support clients that haven't yet caught up with the spec. Given that this was already released, I suppose there's no point in changing it yet again though?

[uhoreg]

Good catch. Thanks