Update XmlReader.ReadElementContentAsInt() exception listing (#2416)

Remove `ArgumentNullException` from the list of `XmlReader.ReadElementContentAsInt()` exceptions, because parameterless methods do not throw argument exceptions.

Author: vslynko

xml/System.Xml/XmlReader.xml

@@ -6356,7 +6356,6 @@ An <see cref="T:System.Xml.XmlReader" /> method was called before a previous asy
  -or-  
 
  The element content cannot be converted to a 32-bit signed integer.</exception>
-        <exception cref="T:System.ArgumentNullException">The method is called with <see langword="null" /> arguments.</exception>
       </Docs>
     </Member>
     <Member MemberName="ReadElementContentAsInt">
@@ -8975,4 +8974,4 @@ This member may behave differently when it is used in a [Portable Class Library]
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>