Summary:
just a minor change - uses the [count api](https://reactjs.org/docs/react-api.html#reactchildrencount)
> Returns the total number of components in children, equal to the number of times that a callback passed to map or forEach would be invoked.
## Changelog
not needed I think
Pull Request resolved: https://github.com/facebook/react-native/pull/28991
Test Plan: tested locally
Differential Revision: D21794081
Pulled By: TheSavior
fbshipit-source-id: bf6d11b2bc854d938aed7268911f89a00bb3f596
Vojtech Novak
92160f3144