FIX: Use new tag routes (#8683)

Commit 1fb7a62 added unambiguous routes for tags. This commit ensures
that the new routes are used.

lib/pretty_text/helpers.rb

@@ -113,7 +113,7 @@ module PrettyText
         [category.url_with_id, text]
       elsif (!is_tag && tag = Tag.find_by(name: text)) ||
             (is_tag && tag = Tag.find_by(name: text.gsub!("#{tag_postfix}", '')))
-        ["#{Discourse.base_url}/tags/#{tag.name}", text]
+        ["#{Discourse.base_url}/tag/#{tag.name}", text]
       else
         nil
       end