docs/fix: Broken link to NCC Group Whitepaper.

[elijahfhopp]

The link to the NCC Group Whitepaper "Assessing Unikernel Security" seems to have moved from https://research.nccgroup.com/wp-content/uploads/2020/07/ncc_group-assessing_unikernel_security.pdf to https://www.nccgroup.com/media/tgsj1css/_ncc_group-assessing_unikernel_security.pdf.

This PR would fix all (only one, actually) occurrences of the broken link.

I have not verified that they are exactly the same, as the old link is 404-ing. But based on the URL and content, I am pretty sure they are the same.