better factorial code added (c++)

[yahya-mohammed07]

Checklist:

• [x] 4 space indentation.
• [x] Coding conventions are followed.
• [x] Input is taken dynamically.
• [x] Sample Input / Output is added at the end of file.
• [x] Logic Documentation (Comments).
• [x] File names are correct.

Changes proposed in this pull request:

• better factorial code

Languages Used:

• c++

Thanks!