docs(examples): increase httpd ProxyTimeout value

With a value that is too small, the HTTP long-polling request receives
an HTTP 502 response code and the connection gets closed.

examples/cluster-httpd/httpd.conf

@@ -51,4 +51,4 @@ RewriteRule /(.*) balancer://nodes_ws/$1 [P,L]
 RewriteCond %{HTTP:Upgrade} !=websocket [NC]
 RewriteRule /(.*) balancer://nodes_polling/$1 [P,L]
 
-ProxyTimeout 3
+ProxyTimeout 60 # must be bigger than pingInterval (25s by default) + pingTimeout (20s by default)