[Tails-dev] Tiny link fix

Hi,

as reported by a user on support channel, a link in the kernel hardening
design doc was non working anymore:

this link:
https://github.com/torvalds/linux/blob/master/Documentation/security/self-protection.txt

from https://tails.boum.org/contribute/design/#index58h3

The fix was easy (replace.txt by .rst) and is to review/merge from:

mercedes508 design/index58h3 cce6d5b8f5

Cheers.