I have followed the webmin instructions to setup Apache and Webmin so that Webmin can be accessed via port 80.
Specifically, I have added this to httpd.conf
ProxyPass /webmin/ http://localhost:10000/
ProxyPassReverse /webmin/ http://localhost:10000/
allow from all
I have added this to /etc/webmin/config
Here is the error that shows up in the Apache log:
File does not exist: /usr/local/apache2/htdocs/webmin
Fixed my own problem. A / is required at the end of the address entered into the browser.