Bug in Matthews_correlation_coefficient

[del2z]

I've tested the ConfusionMatrix and got results similar to the one in https://en.wikipedia.org/wiki/Matthews_correlation_coefficient. However, the code in line 23 is incorrect according to the confusion matrix.
tk, defined as the number of times class k truly occurred, should be the summation along axis=1; pk, on the other hand, is the summation of confusion matrix along axis=0.
Although this mistake does not affect the final result of Matthews correlation coefficient, it may lead to some misunderstanding of the confusion matrix. Hope it can be fixed.

[github-actions]

Hi! thanks for your contribution!, great first issue!

[SkafteNicki]

@del2z Thanks for bringing this to our attention. I created PR #321 to fix this.