Update the `Open a Pull Request` link

https://help.github.com/articles/sending-pull-requests no longer works:

```bash 
curl -IsS https://help.github.com/articles/sending-pull-requests

HTTP/1.1 404 Not Found
Cache-Control: no-cache
Content-Type: text/html; charset=utf-8
Date: Tue, 26 Feb 2013 10:21:51 GMT
Set-Cookie: _help_session=BAh7B0kiD3Nlc3Npb25faWQGOgZFRkkiJWUxYTlhMTU0ZjEzODE2Y2FjY2Q4ZmExZjFjZTVkMjczBjsAVEkiEF9jc3JmX3Rva2VuBjsARkkiMURVYWJ4cFFnMkZldTRMTi9yL01QRTVJN0pWVTZ1VEYzS3B2cytsUlpZaGM9BjsARg%3D%3D--d0b47b64e066e3968141045e1812b71378cbaa80; path=/; expires=Sun, 26-Feb-2023 10:21:51 GMT; secure; HttpOnly
Status: 404 Not Found
Strict-Transport-Security: max-age=31536000
X-Rack-Cache: miss
X-Request-Id: 8184f4b0a776686817f7ea711d9888db
X-Runtime: 0.014394
X-Ua-Compatible: IE=Edge,chrome=1
Connection: keep-alive
```