updating DIRECTORY.md

Author: cclauss

DIRECTORY.md

@@ -791,6 +791,7 @@
   * [Sumset](maths/sumset.py)
   * [Sylvester Sequence](maths/sylvester_sequence.py)
   * [Tanh](maths/tanh.py)
+  * [Test Factorial](maths/test_factorial.py)
   * [Test Prime Check](maths/test_prime_check.py)
   * [Three Sum](maths/three_sum.py)
   * [Trapezoidal Rule](maths/trapezoidal_rule.py)