man7 link should be replaced

[Artoria2e5]

man7 hosts a very outdated version of manual pages from the linux man-pages project, among other sources. mkerrisk/man-pages#33

There are sites that provide up-to-date manpages, as suggested in mkerrisk/man-pages#33 (comment). Compare:

• https://man7.org/linux/man-pages/man4/urandom.4.html
• https://manned.org/urandom.4
• https://www.mankier.com/4/urandom

(At the time of writing the pages are the same. Which is kind of my fault, because I was supposed to follow through on my patch to update them after the big Linux RNG changes.)