doc(reverse-proxy): use http2 directive

Avoid 

```
nginx: [warn] the "listen ... http2" directive is deprecated, use the "http2" directive instead in
```

REF: https://nginx.org/en/docs/http/ngx_http_v2_module.html

docs/admin/reverse-proxy.md

@@ -114,8 +114,9 @@ listen [::]:80;
 to
 
 ```nginx
-listen 443 ssl http2;
+listen 443 ssl;
-listen [::]:443 ssl http2;
+listen [::]:443 ssl;
+http2 on;
 ```
 
 **Add the SSL certificate information**