docs: update DIRECTORY.md

Author: github-actions

DIRECTORY.md

@@ -255,6 +255,7 @@
   * [Angka Proth](https://github.com/bellshade/Python/blob/main/math/angka_proth.py)
   * [Bisection Math](https://github.com/bellshade/Python/blob/main/math/bisection_math.py)
   * [Convolution](https://github.com/bellshade/Python/blob/main/math/convolution.py)
+  * [Cos](https://github.com/bellshade/Python/blob/main/math/cos.py)
   * [Counter](https://github.com/bellshade/Python/blob/main/math/counter.py)
   * [Digit Sum](https://github.com/bellshade/Python/blob/main/math/digit_sum.py)
   * [Exponential Function](https://github.com/bellshade/Python/blob/main/math/exponential_function.py)