Fix Checkstyle XML.
diff --git a/core/checkstyle-suppressions.xml b/core/checkstyle-suppressions.xml
index d385aaf..1464f47 100644
--- a/core/checkstyle-suppressions.xml
+++ b/core/checkstyle-suppressions.xml
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE suppressions PUBLIC
-     -//Checkstyle//DTD SuppressionFilter Configuration 1.0//EN
+     "-//Checkstyle//DTD SuppressionFilter Configuration 1.0//EN"
      "https://checkstyle.org/dtds/suppressions_1_0.dtd">
 <!--
    Licensed to the Apache Software Foundation (ASF) under one or more