[minor] Fix nit

Author: lpinca

test/websocket.test.js

@@ -26,7 +26,7 @@ describe('WebSocket', () => {
       );
     });
 
-    it('accepts `url.URL` objects as url', function (done) {
+    it('accepts `url.URL` objects as url', (done) => {
       const agent = new CustomAgent();
 
       agent.addRequest = (req, opts) => {