Fix Checkstyle suppressions DTD link.

src/conf/checkstyle-suppressions.xml

@@ -17,8 +17,8 @@ limitations under the License.
 -->
 
 <!DOCTYPE suppressions PUBLIC
-    "-//Puppy Crawl//DTD Suppressions 1.1//EN"
-    "http://www.puppycrawl.com/dtds/suppressions_1_1.dtd">
+    "-//Checkstyle//DTD SuppressionFilter Configuration 1.0//EN"
+    "https://checkstyle.org/dtds/suppressions_1_0.dtd">
 <!-- SEE: http://checkstyle.sourceforge.net/config.html#Examples -->
 <suppressions>
     <suppress checks="LineLength"  files="InstructionFinder.java"/> <!-- TODO later -->