Tweak log message a bit

SERVER and CLIENT is a bit hard to understand.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198721870

java/google/registry/proxy/ProxyServer.java

@@ -125,7 +125,7 @@ public class ProxyServer implements Runnable {
                 // This setter also calls channel.read() to request read operation.
                 inboundChannel.config().setAutoRead(true);
                 logger.atInfo().log(
-                    "Relay established: %s <-> %s\nSERVER: %s\nCLIENT: %s",
+                    "Relay established: %s <-> %s\nFRONTEND: %s\nBACKEND: %s",
                     inboundProtocol.name(),
                     outboundProtocol.name(),
                     inboundChannel,