[docs] Remove trailing / from proxy_pass for nginx (#1077)

The trailing / can break relative URLs.

docs/installation_guide/nginx.md

@@ -61,7 +61,7 @@ server {
   listen [::]:80;
   server_name example.org;
   location / {
-    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";