Fix link in sick pay calculator

Had a double ( which broke the resulting link
Fix broken link in paternity-leave

Was pointing at the old guides that have since been merged.  One of them
was 404ing
Fix broken links in vehicles you can drive

Some had a trailing comma

Also update links to the overview of guides to link to the root of the
guide as this is going to become the canonical source shortly.
Merge pull request #495 from alphagov/fix_broken_links

Fix some broken links