Fix external 404 URL at /rest-api/using-the-rest-api/authentication/

[jasalt]

Anchor url https://ec.haxx.se/http/http-auth for "here’s the documentation for how to use it with cURL" gives 404 at https://developer.wordpress.org/rest-api/using-the-rest-api/authentication/.

[pjohanneson]

The URL should (probably) be replaced with https://ec.haxx.se/libcurl-http/auth.html.