Update DTD

Author: garydgregory

src/conf/checkstyle.xml

@@ -16,8 +16,8 @@ See the License for the specific language governing permissions and
 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">
 <module name="Checker">
   <property name="localeLanguage" value="en" />
   <module name="JavadocPackage">