Fix link to XML Encryption standard (dotnet#8593)

Fixes dotnet#6754

Author: wenz

xml/ns-System.Security.Cryptography.Xml.xml

@@ -1,6 +1,6 @@
 <Namespace Name="System.Security.Cryptography.Xml">
   <Docs>
-    <summary>Contains classes to support the creation and validation of XML digital signatures. The classes in this namespace implement the World Wide Web Consortium Recommendation, <see href="https://www.w3.org/TR/xmldsig-core/">XML-Signature Syntax and Processing</see>.</summary>
+    <summary>Contains classes to support the creation and validation of XML digital signatures. The classes in this namespace implement the World Wide Web Consortium Recommendation, <see href="https://www.w3.org/TR/2002/REC-xmlenc-core-20021210/">XML-Signature Syntax and Processing</see>.</summary>
     <remarks>To be added.</remarks>
   </Docs>
 </Namespace>