Update the Agent => Master security link to point to a non-Apache redirect

This help text points to a very old redirect which is only editable in our
infrastructure Puppet code (which will at some point in the distant future be
removed).

Changing to jenkins.io/redirect means that the destination of this redirect
is easily modified by pull requests to
https://github.com/jenkins-infra/jenkins.io

See also jenkins-infra/jenkins.io#520