Fix typo in nginx config

Author: garryod

backstage/container_utils/default.conf.template

@@ -14,6 +14,6 @@ server {
 
     error_page 500 502 503 504 /50x.html;
     location = /50x.html {
-        root /usr/share/ngixn/html;
+        root /usr/share/nginx/html;
     }
 }