Wording improvement for Guide chapter 8

In
https://www.netbsd.org/docs/guide/en/chap-cons.html

You can change the compiled in kernel default
->
You can change the compiled-in kernel default

There are also a couple http: URLs to www.netbsd.org that could be
https: URLs instead, in links.  If it's assumed the Guide is always
mirrored with the rest of the site, they could be made relative links.