fix api link (#2255)

Author: mairaw

xml/System/ValueTuple`3.xml

@@ -782,7 +782,7 @@ The <xref:System.ValueTuple%603> type is not serializable in .NET Core 1.x or in
       <Parameters />
       <Docs>
         <summary>Returns a string that represents the value of this <see cref="T:System.ValueTuple`3" /> instance.</summary>
-        <returns>The string representation of this <see cref="T:System.ValueTuple`2" /> instance.</returns>
+        <returns>The string representation of this <see cref="T:System.ValueTuple`3" /> instance.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
@@ -794,4 +794,4 @@ The <xref:System.ValueTuple%603> type is not serializable in .NET Core 1.x or in
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>