Fixes #171, correct links in update guide 5.1 to 6.0
Fix typos in CONTRIBUTING.md
Merge branch 'fix-typo' into 'master'

Fix typos in CONTRIBUTING.md

This MR fixes two typos in [CONTRIBUTING.md](CONTRIBUTING.md).
Merge branch 'doc/fix_update_doc_links' into 'master'

Fix links in Update guide 5.1 to 6.0

Links have missing slashes and additionally point to `master` branch.
This MR prevents a 404 error (missing slash) and points to the correct branch (6-0-stable) for comparison of the config files.

/cc @sytse
Merge branch 'master' of gitlab.com:gitlab-org/gitlab-ce