Fix Checkstyle configuration DTD link.

Author: garydgregory

checkstyle.xml

@@ -17,8 +17,8 @@ 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">
 
 <!-- commons lang customization of default Checkstyle behavior -->
 <module name="Checker">