Fix TLS error in acmregister config

With the Shibboleth verifier URL set to "https://my.fullerton.edu", the
bot yields a TLS error. We can replicate this error (as of the time of
this commit) by running `curl -Lv https://my.fullerton.edu`:

    ―❤―▶ curl -Lv https://my.fullerton.edu
    * Host my.fullerton.edu:443 was resolved.
    * IPv6: (none)
    * IPv4: 137.151.127.110
    *   Trying 137.151.127.110:443...
    * Connected to my.fullerton.edu (137.151.127.110) port 443
    * ALPN: curl offers h2,http/1.1
    * TLSv1.3 (OUT), TLS handshake, Client hello (1):
    * TLSv1.3 (IN), TLS handshake, Server hello (2):
    * TLSv1.3 (IN), TLS handshake, Encrypted Extensions (8):
    * TLSv1.3 (IN), TLS handshake, Certificate (11):
    * TLSv1.3 (OUT), TLS alert, unknown CA (560):
    * OpenSSL/3.0.13: error:16000069:STORE routines::unregistered scheme
    * Closing connection
    curl: (35) OpenSSL/3.0.13: error:16000069:STORE routines::unregistered scheme

Instead, we switch to "https://csufullerton.instructure.com" which is
the URL to the Canvas instance at CSUF. This URL seems to be configured
properly and does the same job as the previous URL.