Create fermats_little_theorem.cpp

[bhavyammodi]

Description

Adding c++ code for Fermats Little Theorem.

Type of change

• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [x] My code follows the style guidelines(Clean Code) of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have created a helpful and easy to understand README.md
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests/screenshots(if any) that prove my fix is effective or that my feature works