Fix proxying API request docs (facebook#8515)

The http-proxy-middleware package has made a major version upgraded and
introduced a breaking change. This fixes the "Configuring the Proxy
Manually" documentation to match the latest version.

docusaurus/docs/proxying-api-requests-in-development.md

@@ -85,7 +85,7 @@ $ yarn add http-proxy-middleware
 Next, create `src/setupProxy.js` and place the following contents in it:
 
 ```js
-const proxy = require('http-proxy-middleware');
+const { createProxyMiddleware } = require('http-proxy-middleware');
 
 module.exports = function(app) {
   // ...
@@ -95,12 +95,12 @@ module.exports = function(app) {
 You can now register proxies as you wish! Here's an example using the above `http-proxy-middleware`:
 
 ```js
-const proxy = require('http-proxy-middleware');
+const { createProxyMiddleware } = require('http-proxy-middleware');
 
 module.exports = function(app) {
   app.use(
     '/api',
-    proxy({
+    createProxyMiddleware({
       target: 'http://localhost:5000',
       changeOrigin: true,
     })