docs: avoid DNS request loop

Fix the example config for the dns upstreams.

(cherry picked from commit ffeaf4451e7c79738dad66fb850fc0345578b050)

docs/features/dns-cache.rst

@@ -40,7 +40,7 @@ some browsers would open the searchpage instead.
 
   dns = {
     cacheentries = 5000,
-    servers = { '2001:db8::1', },
+    servers = { '2001:4860:4860::8888', '2001:4860:4860::8844' },
   },
 
   next_node = {