Add docstring for deficient number explanation

Added a docstring explaining deficient numbers and provided examples.

Author: cclauss

maths/special_numbers/deficient_numbers.py

@@ -3,6 +3,8 @@
 the sum of its proper divisors is less than the number itself.
 
 Examples of Deficient Numbers: 1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 13, ...
+
+https://en.wikipedia.org/wiki/Deficient_number
 """