Update example to use multiple upstream DNS servers

Update example to use multiple upstream DNS servers

Author: satishweb

Readme.md

@@ -58,7 +58,7 @@ sudo make uninstall
 ```bash
 docker run -d --name doh-server \
   -p 8053:8053 \
-  -e UPSTREAM_DNS_SERVER="udp:8.8.8.8:53" \
+  -e UPSTREAM_DNS_SERVER="udp:208.67.222.222:53,udp:208.67.220.220:53" \
   -e DOH_HTTP_PREFIX="/dns-query" \
   -e DOH_SERVER_LISTEN=":8053" \
   -e DOH_SERVER_TIMEOUT="10" \