Fix Checkstyle configuration DTD link.
diff --git a/checkstyle.xml b/checkstyle.xml
index 487384f..ae00b8f 100644
--- a/checkstyle.xml
+++ b/checkstyle.xml
@@ -16,8 +16,8 @@
    limitations under the License.
 -->
 <!DOCTYPE module PUBLIC
-      "-//Puppy Crawl//DTD Check Configuration 1.1//EN"
-      "http://www.puppycrawl.com/dtds/configuration_1_1.dtd">
+      "-//Checkstyle//DTD Checkstyle Configuration 1.2//EN"
+      "https://checkstyle.org/dtds/configuration_1_2.dtd">
   
   <!--
     Checkstyle checks configured for Maven.