docs: update asynchronous example

[is2ei]

Closes #137

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

example request:

$ curl -H  "origin:localhost:3000"  -v http://localhost:3000/
*   Trying 127.0.0.1:3000...
* Connected to localhost (127.0.0.1) port 3000 (#0)
> GET / HTTP/1.1
> Host: localhost:3000
> User-Agent: curl/7.77.0
> Accept: */*
> origin:localhost:3000
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< vary: Origin
< content-type: application/json; charset=utf-8
< content-length: 17
< Date: Sun, 29 May 2022 04:46:53 GMT
< Connection: keep-alive
< Keep-Alive: timeout=5
<
* Connection #0 to host localhost left intact
{"hello":"world"}

[mcollina]

lgtm

[Fdawgs]

Suggested change

	if (/localhost/.test(req.headers.origin)) {
	if (/^localhost$/m.test(req.headers.origin)) {

Otherwise compromised-website.com/localhost would pass this.

[Fdawgs]

Suggested change

	origin: true
	// This is NOT recommended for production as it enables reflection exploits
	origin: true