Fix example alias URL

etc/nginx.conf.sample

@@ -35,7 +35,7 @@ server {
     location /static/CACHE/ {
         access_log off;
         expires 864000;
-        alias /home/mywagtail/static/CACHE/;
+        alias /home/mywagtail/app/static/CACHE/;
     }
 
     # Only server /media/images by default, not e.g. original_images/.