Fix miss `%s` bug (#17)

 miss `%s` format for `expected` value
diff --git a/validator/src/main/java/org/apache/skywalking/plugin/test/agent/tool/validator/assertor/exception/TagValueNotEqualsException.java b/validator/src/main/java/org/apache/skywalking/plugin/test/agent/tool/validator/assertor/exception/TagValueNotEqualsException.java
index 6c8d41f..4517207 100644
--- a/validator/src/main/java/org/apache/skywalking/plugin/test/agent/tool/validator/assertor/exception/TagValueNotEqualsException.java
+++ b/validator/src/main/java/org/apache/skywalking/plugin/test/agent/tool/validator/assertor/exception/TagValueNotEqualsException.java
@@ -32,6 +32,6 @@
 
     @Override
     public String getCauseMessage() {
-        return String.format("[tag(%s) value]: expected=>{}, actual=>{%s}\n", tagKey, expected, actual);
+        return String.format("[tag(%s) value]: expected=>{%s}, actual=>{%s}\n", tagKey, expected, actual);
     }
 }