updating DIRECTORY.md

Author: github-actions

DIRECTORY.md

@@ -88,6 +88,7 @@
   * [FindHcf](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/FindHcf.js)
   * [FindLcm](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/FindLcm.js)
   * [GridGet](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/GridGet.js)
+  * [MeanSquareError](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/MeanSquareError.js)
   * [ModularBinaryExponentiationRecursive](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/ModularBinaryExponentiationRecursive.js)
   * [Palindrome](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/Palindrome.js)
   * [PascalTriangle](https://github.com/TheAlgorithms/Javascript/blob/master/Maths/PascalTriangle.js)