[now dev] Fix validate functions config failed in now json (#3414)
[now dev] Fix validate functions config failed in now json (#3414)

Follow up to #3408 .

```
> Error! Checking for updates failed
> Now CLI 16.6.3 dev (beta) — https://zeit.co/feedback/dev
> Error! Invalid `functions` property: ['api/test.js'] should NOT have additional properties
```
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