fix typo

playground/scripts/postinstall.js

-const cp = (`child_process`);
+const cp = require('child_process');
 
-cp.execSync(`rm yarn.lock `);
+cp.execSync(`rm yarn.lock`);
 cp.execSync(`rm -rf node_modules/react-native-navigation/node_modules`);
 cp.execSync(`rm -rf node_modules/react-native-navigation/playground`);