Fix R2dbcTransactionManager debug log: don't log a Mono (#29800)

When logging the current connection inside R2dbcTransactionManager
doBegin, the mono object was logged instead of the connection lambda
parameter.

Other similar debug-level logs do use the actual Connection object,
so this commit does the same.

spring-r2dbc/src/main/java/org/springframework/r2dbc/connection/R2dbcTransactionManager.java

@@ -200,7 +200,7 @@ public class R2dbcTransactionManager extends AbstractReactiveTransactionManager
 				Mono<Connection> newCon = Mono.from(obtainConnectionFactory().create());
 				connectionMono = newCon.doOnNext(connection -> {
 					if (logger.isDebugEnabled()) {
-						logger.debug("Acquired Connection [" + newCon + "] for R2DBC transaction");
+						logger.debug("Acquired Connection [" + connection + "] for R2DBC transaction");
 					}
 					txObject.setConnectionHolder(new ConnectionHolder(connection), true);
 				});