repo website link 404 - update to correct site

[koolhand]

Great doc. The website link for the repo currently points to a broken link - where the beta version of this doc was once kept https://www.ncsc.gov.uk/blog-post/zero-trust-principles-beta-release

I recommend updating to the finalised doc location https://www.ncsc.gov.uk/collection/zero-trust-architecture

Thanks!