Fix typo in dangerfile.js which results in an unreachable code path… (#32277)

## Summary

Fix typo in dangerfile.js which results in an unreachable code path
which ought to be hit when there is no matching base artifact during
DangerCI automated code review.

See:
https://github.com/facebook/react/blob/221f3002caa2314cba0a62950da6fb92b453d1d0/dangerfile.js#L73
Compare:
https://github.com/facebook/react/blob/221f3002caa2314cba0a62950da6fb92b453d1d0/dangerfile.js#L171
And the case which should hit this code path:
https://github.com/facebook/react/blob/221f3002caa2314cba0a62950da6fb92b453d1d0/dangerfile.js#L160

Given the above context, the condition `Number === Infinity` is clearly
meant to be `decimal === Infinity`, which it will be if the `catch`
statement triggers when there is no matching base artifact. Without this
fix, the primitive value `Infinity` is passed to
`percentFormatter.format(decimal)`, resulting in the string `'+∞%'`.
With this fix, the resulting string will be the intended `'New file'`.

## [Resolves issue
32278](https://github.com/facebook/react/issues/32278)