Fix documentation of http-proxy-middleware (facebook#5226)

Author: banyan

packages/react-scripts/template/README.md

@@ -1211,7 +1211,7 @@ You can now register proxies as you wish! Here's an example using the above `htt
 const proxy = require('http-proxy-middleware');
 
 module.exports = function(app) {
-  app.use(proxy('/api', { target: 'http://localhost:5000/' }));
+  app.use('/api', proxy({ target: 'http://localhost:5000/' }));
 };
 ```