TUTORIAL.md: httpd man page is in section 8

[dijkstracula]

Just a tiny nit.

The current URL to the OpenBSD man page for httpd returns a 200, but with a "No results found" message. The httpd man page is in section 8, so this PR adjusts the URL to point to the right section.

$ curl -s http://man.openbsd.org/httpd.1 | grep "No results" | wc
       1       3      18
$ curl -s http://man.openbsd.org/httpd.8 | grep "No results" | wc
       0       0       0