Fix merge sort remaining elements merge logic

[Yashasvi-dot-V]

Description

This pull request updates the merge_sort implementation in sorts/merge_sort.py.

Changes Made

• Replaced the while loops used to copy the remaining elements from the left and right halves with Python slicing assignment (collection[k:] = ...).
• This simplifies the code while maintaining the same functionality and correctness of the merge step.

Testing

• Ran the existing doctests using python -m doctest -v sorts/merge_sort.py.
• All tests passed successfully.