update README.md

Signed-off-by: Harsh <[email protected]>

Author: hrshya

lib/node_modules/@stdlib/stats/incr/nanmse/README.md

@@ -140,6 +140,8 @@ console.log( accumulator() );
 
 <section class="links">
 
+[mean-squared-error]: https://en.wikipedia.org/wiki/Mean_squared_error
+
 </section>
 
 <!-- /.links -->