YARN-8866. Fix a parsing error for crossdomain.xml.

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-ui/src/main/webapp/public/crossdomain.xml

@@ -18,7 +18,12 @@
 * limitations under the License.
 -->
 
-<!DOCTYPE cross-domain-policy SYSTEM "http://www.adobe.com/xml/dtds/cross-domain-policy.dtd">
+<!DOCTYPE cross-domain-policy [
+  <!ELEMENT cross-domain-policy (site-control)*>
+  <!ELEMENT site-control (#PCDATA)>
+  <!ATTLIST site-control
+          permitted-cross-domain-policies CDATA #REQUIRED>
+  ]>
 <cross-domain-policy>
   <!-- Read this: www.adobe.com/devnet/articles/crossdomain_policy_file_spec.html -->