Fix links to docs from pre filtered Finder

When linking back to a pre-filtered Finder, the URL has a trailing slash
which then breaks links to documents as they are set as
`finder-path/doc-name` which it assumes are then at
`finder-path/finder-path/doc-name`. This commit removes the trailing
slash.
Merge pull request #42 from alphagov/fix-links-to-documents-from-pre-filtered-finder

Fix links to docs from pre filtered Finder