updating DIRECTORY.md

Author: github-actions

DIRECTORY.md

@@ -655,6 +655,7 @@
   * [Sum Of Harmonic Series](maths/sum_of_harmonic_series.py)
   * [Sumset](maths/sumset.py)
   * [Sylvester Sequence](maths/sylvester_sequence.py)
+  * [Tanh](maths/tanh.py)
   * [Test Prime Check](maths/test_prime_check.py)
   * [Trapezoidal Rule](maths/trapezoidal_rule.py)
   * [Triplet Sum](maths/triplet_sum.py)