improve explanation for factorial algorithm

[Vikash565-dot]

Added a clear explanation and example to help beginners understand the algorithm.
This improves readability and documentation quality.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.92%. Comparing base (08d8c6b) to head (8096f6d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1865   +/-   ##
=======================================
  Coverage   85.91%   85.92%           
=======================================
  Files         379      379           
  Lines       19778    19784    +6     
  Branches     3015     3014    -1     
=======================================
+ Hits        16993    16999    +6     
  Misses       2785     2785           

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[Vikash565-dot]

Let me know if any changes are required. Happy to update!