Typo fix + update BuildBox link
I noticed the link to the BuildBox repo takes the user to buildbox-fuse, so updated it to be the main BuildBox group.
Changes proposed in this merge request:
- Change link from buildstream/buildbox to buildgrid/buildbox
- Minor typo fix, "BuildGrid webside" -> "BuildGrid website" (updated in link section as well as text).
Check the links go where they should.