fixing the broken link in document-checker
tidy up legalisation-document-checker markdown
fixing broken link in state-pension-through-partner
updating broken link for south korea credit card authorisation

When we adopted the WW API country list korea became south-korea but we forgot to update this file. Only found it because the old destination 'korea' page has been removed.
Merge pull request #937 from alphagov/fixing_broken_links

Updating broken links in smart-answers