Summary:
Just small fix for those who are copy & pasting commands from error message.
## Changelog:
[INTERNAL] [CHANGED] - Update erorr message to use `npx` when calling `react-native`
Pull Request resolved: https://github.com/facebook/react-native/pull/42691
Test Plan: _
Reviewed By: cipolleschi
Differential Revision: D53177186
Pulled By: cortinico
fbshipit-source-id: e680cde81fde1f560dfeb1a85c8ad90090d69653