Fix .File use warning

A warning was missed whilst implementing the source file link:

    .File.Path on zero object. Wrap it in if or with: {{ with .File }}{{ .Path }}{{ end }}

Update to use the recommended format.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>

themes/beautifulhugo/layouts/partials/footer.html

@@ -61,7 +61,7 @@
           </ul>
         </p>
         <p class="credits text-center text-muted">
-            <a href="{{ $.Site.Params.sourceLink}}{{ $.File.Path }}">View page source</a>
+            <a href="{{ $.Site.Params.sourceLink}}{{ with .File }}{{ .Path }}{{ end }}">View page source</a>
         </p>
 
         <!-- Please don't remove this, keep my open source work credited :) -->