fix formdata-node example (node-fetch#1068)

Author: samijaber

README.md

@@ -370,11 +370,11 @@ const data = await response.json();
 console.log(data)
 ```
 
-node-fetch also supports spec-compliant FormData implementations such as [formdata-node](https://github.com/octet-stream/form-data):
+node-fetch also supports spec-compliant FormData implementations such as [form-data](https://github.com/form-data/form-data) and [formdata-node](https://github.com/octet-stream/form-data):
 
 ```js
 const fetch = require('node-fetch');
-const FormData = require('form-data');
+const FormData = require('formdata-node');
 
 const form = new FormData();
 form.set('greeting', 'Hello, world!');