Fix broken releases script

Summary:
Fails on my machine due to fact that `replace` returns an instance of a String, rather than an instance of ShellString (that includes `to` on its prototype).

Solution is to use an explicit `writeFileSync`. You can see that change in the wild on 0.50-stable branch.

CC janicduplessis (edit by hramos)
Closes https://github.com/facebook/react-native/pull/16303

Differential Revision: D6031331

Pulled By: hramos

fbshipit-source-id: 41c583d53df75bea1a55fa19174d912e414209c0