Returning index instead of boolean when knuth_morris_pratt matches.

In [knuth morris pratt algorithm](https://github.com/TheAlgorithms/Python/blob/master/strings/knuth_morris_pratt.py), I think it's better to return the "index" of the matched substring in case of success, and `-1` in case of failure, so that it would be consistent with the `str.find()` in Python. `True` and `False` aren't that helpful.