I want to add FibonacciNumber c++ code

[TakshalModi02]

This issue is about [FibonacciNumber ].

• [X] Issue Title is in form FibonacciNumber [C++].
• [X] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
• [X] This Algo/DS is worth adding to this repository.
• [X] Assign this issue to me (I want to work on this).
  • [X] I ensure that I am not already assigned to 2 or more issues.
• [X] I will send my PR only after I'm assigned to this issue by any maintainer.
• [X] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

[isar-hub]

assign this issue to me