Use correct endpoints for cURL examples

Requests to the https://ona.io/api/v1 endpoint are redirected to http://ona.io/api/v1, which is subsequently redirected to http://ona.io/api/v1/. A default cURL configuration won't show you the 301 status code, and doesn't follow redirects by default.
Restore https

Turns out a trailing slash was all that was needed.
Merge pull request #484 from okal/patch-1

Use correct endpoints for cURL examples