Fixing typo missed in dotnet#2912

Author: jozkee

xml/System.Numerics/Complex.xml

@@ -2006,7 +2006,7 @@ Languages that do not support custom operators can call the <xref:System.Numeric
 
           ]]></format>
         </remarks>
-        <altmember cref="M:System.Numerics.Complex.Add(System.NDouble,System.Numerics.Complex)" />
+        <altmember cref="M:System.Numerics.Complex.Add(System.Double,System.Numerics.Complex)" />
       </Docs>
     </Member>
     <Member MemberName="op_Addition">