[docs] changing references of "React Native packager" to "Metro bundler"

Summary: As suggested in D2612, changing references of "React Native packager" to "Metro bundler".

Test Plan: N/A

Reviewers: atul, ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, boristopalov

Differential Revision: https://phabricator.ashoat.com/D2687