Fix v4v6 option in HAProxy example config (#4790)

The v4v6 option only has a usage one ipv6 socket: https://serverfault.com/q/747895

Signed-off-by: Flakebi <flakebi@t-online.de>

changelog.d/4790.bugfix

@@ -0,0 +1 @@
+Fix v4v6 option in HAProxy example config. Contributed by Flakebi.

docs/reverse_proxy.rst

@@ -88,16 +88,14 @@ Let's assume that we expect clients to connect to our server at
 * HAProxy::
 
       frontend https
-        bind 0.0.0.0:443 v4v6 ssl crt /etc/ssl/haproxy/ strict-sni alpn h2,http/1.1
-        bind :::443 ssl crt /etc/ssl/haproxy/ strict-sni alpn h2,http/1.1
+        bind :::443 v4v6 ssl crt /etc/ssl/haproxy/ strict-sni alpn h2,http/1.1
 
         # Matrix client traffic
         acl matrix hdr(host) -i matrix.example.com
         use_backend matrix if matrix
 
       frontend matrix-federation
-        bind 0.0.0.0:8448 v4v6 ssl crt /etc/ssl/haproxy/synapse.pem alpn h2,http/1.1
-        bind :::8448 ssl crt /etc/ssl/haproxy/synapse.pem alpn h2,http/1.1
+        bind :::8448 v4v6 ssl crt /etc/ssl/haproxy/synapse.pem alpn h2,http/1.1
         default_backend matrix
 
       backend matrix