Update outdated URLs

Created by: nwallace

While reading the book, I noticed the link to Plan9 was broken. It appears that the original website is down indefinitely. I confirmed in the #plan9 channel on freenode that the mirror at 9p.io is considered the canonical site for the project now (see screenshot for IRC conversation).

While I was at it, I updated a couple other links that have moved permanently (libmalloc -> ralloc and tfs).

There are two other broken links: the links to https://github.com/EGhiorzi (appears here and here), but I don't know what those URLs should be to fix them.