Update DTD

Author: garydgregory

src/site/resources/checkstyle/checkstyle.xml

@@ -17,8 +17,8 @@ limitations under the License.
 -->
 
 <!DOCTYPE module PUBLIC
-    "-//Checkstyle//DTD Checkstyle Configuration 1.2//EN"
-    "https://checkstyle.org/dtds/configuration_1_2.dtd">
+    "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+    "https://checkstyle.org/dtds/configuration_1_3.dtd">
 
 <!-- commons codec customization of default Checkstyle behavior -->
 <module name="Checker">