create armstrong number.cpp

[mishrasushruti99]

i have proposed to program that checks whether or not the number is an armstrong or not ie sum of cubes of the digits of the number is equal to the number itself.

Fixes #

Checklist

• I have read the [Contribution Guide] and my PR follows them.
• My branch is up-to-date with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• The contents added are completelt original and have no copyright issues.

Short description of what this resolves:

An Armstrong number of three digits is an integer such that the sum of the cubes of its digits is equal to the number itself. For example, 371 is an Armstrong number since 33 + 73 + 1**3 = 371.

Changes proposed in this pull request:

-creation of a program which checks whther or not a number is an armstrong number.