Change proxy `DefaultRelayURL` back to snowflake.torproject.net

!59 (merged) (2021-10-14) changed DefaultBrokerURL and DefaultRelayURL from *.bamsoftware.com to *.torproject.net. But commit 50e4f4fd (2021-10-25) changed them back. Commit 0e8d41ba (2021-10-26) fixed DefaultBrokerURL but left DefaultRelayURL pointing at snowflake.bamsoftware.com.

The documentation for -relay in proxy/README.md also needs to be updated (cf. #40232 (closed)).