security review url on github is dead

[rapus95]

url according to repo readme (https://github.com/DefGuard/defguard) and organization readme (https://github.com/DefGuard) is that: https://defguard.net/images/decap/isec-defguard.pdf

Following the website defguard.net, the correct url most probably is: https://defguard.net/pdf/isec-defguard.pdf