fix(shortcode): remove Hugo 0.55+ deprecation warning for Gallery

Fix #1074

layouts/shortcodes/gallery.html

@@ -50,7 +50,7 @@
   </a>
   {{end}}
   {{else}}
-    {{ errorf "Unable to load gallery in %s." .Page.Filename }}
+    {{ errorf "Unable to load gallery in %s." .Page.File.Filename }}
   {{end}}
   {{end}}
 </div>