fix typo in README.md (facebook#20993)

Summary:
The original facebook is misspelled.
pass all current ci.
none
 [GENERAL] [INTERNAL] [MISC] - fix typo in README.md
Pull Request resolved: facebook#20993

Differential Revision: D9671977

Pulled By: hramos

fbshipit-source-id: 2c279cb681696460801e11012b4a456d5375ce24

README.md

@@ -1,4 +1,4 @@
-# [React Native](https://facebook.github.io/react-native/) ·  [![Circle CI Status](https://circleci.com/gh/facebook/react-native.svg?style=shield)](https://circleci.com/gh/facebook/react-native) [![Build status](https://ci.appveyor.com/api/projects/status/github/facebook/react-native?branch=master&svg=true)](https://ci.appveyor.com/project/facebok/react-native/branch/master) [![npm version](https://badge.fury.io/js/react-native.svg)](https://badge.fury.io/js/react-native) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](CONTRIBUTING.md#pull-requests)
+# [React Native](https://facebook.github.io/react-native/) ·  [![Circle CI Status](https://circleci.com/gh/facebook/react-native.svg?style=shield)](https://circleci.com/gh/facebook/react-native) [![Build status](https://ci.appveyor.com/api/projects/status/github/facebook/react-native?branch=master&svg=true)](https://ci.appveyor.com/project/facebook/react-native/branch/master) [![npm version](https://badge.fury.io/js/react-native.svg)](https://badge.fury.io/js/react-native) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](CONTRIBUTING.md#pull-requests)
 
 Learn once, write anywhere: Build mobile apps with React.