add comment (#5620)

Co-authored-by: 吴晟 Wu Sheng <wu.sheng@foxmail.com>

apm-sniffer/apm-sdk-plugin/sharding-jdbc-1.5.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/sjdbc/ExecuteEventListener.java

@@ -83,6 +83,7 @@ public class ExecuteEventListener {
                 if (event.getException().isPresent()) {
                     span.log(event.getException().get());
                 }
+                // fall through
             case EXECUTE_SUCCESS:
                 ContextManager.stopSpan();
         }