Update for reverse proxying static resources (#17670)

Gitea fetches static resources from /assets, so nginx configuration has to be updated accordingly.

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

docs/content/doc/usage/reverse-proxies.en-us.md

@@ -80,7 +80,7 @@ server {
     listen 80;
     server_name git.example.com;
 
-    location /_/static {
+    location /_/static/assets {
         alias /path/to/gitea/public;
     }