Fix CLI grammar (facebook#1945)

I know this is a very minor thing, but correct grammar makes it easier to understand.

Author: ColinEberhardt

packages/create-react-app/createReactApp.js

@@ -256,7 +256,7 @@ function run(
   const packageToInstall = getInstallPackage(version);
   const allDependencies = ['react', 'react-dom', packageToInstall];
 
-  console.log('Installing packages. This might take a couple minutes.');
+  console.log('Installing packages. This might take a couple of minutes.');
   getPackageName(packageToInstall)
     .then(packageName => checkIfOnline(useYarn).then(isOnline => ({
       isOnline: isOnline,