Fixed various typos

The original link, slackware.org, resulted in a 503 error. I changed the link to slackware.com; I am unsure if this is the correct link for the Slackware distribution, so I will leave this up to your judgment.

For the Lab 7 README.md, there was a typo where the instructions were saying to rename a file to itself, when two different filenames were present.

Lab 8 README.md: changed "elsse's" to "else's".