Rename maths/binary_exponentiation_3.py (#9656)

* updating DIRECTORY.md

* Rename binary_exponentiation_3.py

Rename binary_exponentiation_3.py to binary_exponentiation_2.py because
the original binary_exponentiation_2.py was renamed to
binary_multiplication.py in PR #9513

* updating DIRECTORY.md

* updating DIRECTORY.md

* updating DIRECTORY.md

* updating DIRECTORY.md

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