Only add a canonical tag to the latest version of man pages

The `/v2.6/man/` and `/v2.5/man/` urls actually serve different content,
so they should not all have the same canonical URL. Only make `/man/` the
canonical page of the latest version, `/v2.6/man/`.

source/layouts/two_column_layout.haml

@@ -1,7 +1,7 @@
 - is_command = command?(current_page.url) || whats_new?(current_page.url)
 - v = current_page.url.scan(/v\d\.\d+/).first || current_version
 
-- if /v\d\.\d+\//.match?(current_page.destination_path)
+- if current_page.destination_path.start_with?(current_version)
   - content_for(:canonical, commands_toplevel_path(current_page.destination_path))
 
 ~ wrap_layout :base do