Update fibonacci_sum.cpp

[utkarshbhola]

changed the matrix addition function for easier readability and added a new test case where n =20 and m =30 to calculate the Fibonacci numbers from the 20th Fibonacci number (F(20)) to the 30th Fibonacci number (F(30)).This test case illustrates the capability of your fiboSum function to handle a more significant range of Fibonacci numbers and provides a meaningful result.

Description of Change

Checklist

• [x] Added description of change
• [x] Added file name matches File name guidelines
• [x] Added tests and example, test must pass
• [ ] Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• [x] Relevant documentation/comments is changed or added
• [x] PR title follows semantic commit guidelines
• [x] Search previous suggestions before making a new one, as yours may be a duplicate.
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