fix link to sort icon (#402)

Author: davidhunter08

docs/_includes/layouts/icons-example.njk

@@ -140,7 +140,7 @@
     <tr>
       <td>{{ nhsappIcon('sort') }}</td>
       <td>sort</td>
-      <td><a target="_blank" rel="noopener noreferrer" href="https://github.com/nhsuk/nhsapp-frontend/tree/main/src/assets/icons/messages-unread.svg">sort.svg</a></td>
+      <td><a target="_blank" rel="noopener noreferrer" href="https://github.com/nhsuk/nhsapp-frontend/blob/main/src/assets/icons/sort.svg">sort.svg</a></td>
       <td>Sort a list</td>
     </tr>
     <tr>