README: Use HTTPS URL for docs.ejabberd.im

README: Use HTTPS URL for docs.ejabberd.im

Directly use the HTTPS URL.

    $ curl -I http://docs.ejabberd.im
    HTTP/1.1 301 Moved Permanently
    Server: Cowboy
    Connection: keep-alive
    Location: https://docs.ejabberd.im/
    Date: Mon, 12 Nov 2018 14:31:07 GMT
    Content-Type: text/plain; charset=utf-8
    Via: 1.1 vegur

    $ curl -I https://docs.ejabberd.im
    HTTP/1.1 200 OK
    Server: Cowboy
    Connection: keep-alive
    Set-Cookie: main-session=MTU0MjAzMzA1NnxOd3dBTkZWWVNrbEVRVkUwUVZvM1JGcEtTakpKTTA1RlYxQk1TelZKTmxkUlNGRk5Sa0ZVV0VSS1dVNHpSMDh6VEUxSVJFTkRVMEU9fHnVvedfnvRp4MtnGBKYfXIDSBTPilUvIp6Kz559FNKg; Path=/; Expires=Mon, 12 Nov 2018 22:30:56 GMT; Max-Age=28800; HttpOnly
    Date: Mon, 12 Nov 2018 14:30:56 GMT
    Content-Type: text/html; charset=utf-8
    Via: 1.1 vegur