Correct url for otp in sample.config

The old url used in a comment in the example config for ocserv (https://code.google.com/p/mod-authn-otp/wiki/UsersFile) is not a good source for information about otp auth mod (mainly because of destroyed formatting and google code project is disabled anyway). New url could be: https://github.com/archiecobbs/mod-authn-otp/wiki/UsersFile