[bugfix] Fix proxy_pass in named location (#1794)

A proxy_pass in a named location, @name, should not include a trailing
slash.

docs/installation_guide/advanced.md

@@ -402,7 +402,7 @@ server {
   }
 
   location @fileserver {
-    proxy_pass http://localhost:8080/;
+    proxy_pass http://localhost:8080;
     proxy_set_header Host $host;
     proxy_set_header Upgrade $http_upgrade;
     proxy_set_header Connection "upgrade";