Fix instructions for serving with now (facebook#2052)

* Fix instructions for serving with now

Not sure if new, but the `--single` flag is now important.

* Keep it shorter

Author: davidascher

packages/react-scripts/template/README.md

@@ -1516,7 +1516,7 @@ When you build the project, Create React App will place the `public` folder cont
 3. Add this line to `scripts` in `package.json`:
 
     ```
-    "now-start": "serve build/",
+    "now-start": "serve -s build/",
     ```
 
 4. Run `now` from your project directory. You will see a **now.sh** URL in your output like this: