listen http2 deprecated nginx (updating documentation) (#16831)

More info [here](https://www.nginx.com/blog/nginx-plus-r30-released/).

Nginx threw error's at me when I used all the options of the doc

changelog.d/16831.doc

@@ -0,0 +1 @@
+NGINX listen http2 deprecation in documentation template for reverse proxy.

docs/reverse_proxy.md

@@ -58,12 +58,12 @@ reverse proxy is using.
 
 ```nginx
 server {
-    listen 443 ssl http2;
-    listen [::]:443 ssl http2;
+    listen 443 ssl;
+    listen [::]:443 ssl;
 
     # For the federation port
-    listen 8448 ssl http2 default_server;
-    listen [::]:8448 ssl http2 default_server;
+    listen 8448 ssl default_server;
+    listen [::]:8448 ssl default_server;
 
     server_name matrix.example.com;