lint(spanner): javadoc fixes.

Author: pratickchokhani

google-cloud-spanner/src/main/java/com/google/cloud/spanner/BatchTransactionId.java

@@ -65,8 +65,8 @@ public boolean isMultiplexedSession() {
   @Override
   public String toString() {
     return String.format(
-        "transactionId: %s, sessionId: %s, timestamp: %s",
-        transactionId.toStringUtf8(), sessionId, timestamp);
+        "transactionId: %s, sessionId: %s, timestamp: %s, isMultiplexedSession: %s",
+        transactionId.toStringUtf8(), sessionId, timestamp, isMultiplexedSession);
   }
 
   @Override