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 {
+    location /_/static/assets/ {
-        alias /path/to/gitea/public;
+        alias /path/to/gitea/public/;
     }
 
     location / {
@@ -120,8 +120,8 @@ server {
     listen 80;
     server_name cdn.example.com;
 
-    location /gitea {
+    location /gitea/ {
-        alias /path/to/gitea/public;
+        alias /path/to/gitea/public/;
     }
 
     location / {