Fix alias traversal (#20076)

see https://github.com/yandex/gixy/blob/master/docs/en/plugins/aliastraversal.md

Co-authored-by: 6543 <6543@obermui.de>

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

@@ -88,8 +88,8 @@ server {
     listen 80;
     server_name git.example.com;
 
-    location /_/static/assets {
-        alias /path/to/gitea/public;
+    location /_/static/assets/ {
+        alias /path/to/gitea/public/;
     }
 
     location / {
@@ -120,8 +120,8 @@ server {
     listen 80;
     server_name cdn.example.com;
 
-    location /gitea {
-        alias /path/to/gitea/public;
+    location /gitea/ {
+        alias /path/to/gitea/public/;
     }
 
     location / {
@@ -362,4 +362,4 @@ gitea:
     - "traefik.http.services.gitea-websecure.loadbalancer.server.port=3000"
 ```
 
-This config assumes that you are handling HTTPS on the traefik side and using HTTP between Gitea and traefik.
+This config assumes that you are handling HTTPS on the traefik side and using HTTP between Gitea and traefik.