Fix Information Gain Calculation in Decision Tree Algorithm

[ElmekaouiHaitham]

This pull request corrects the formula for calculating Information Gain in the decision tree algorithm. The previous formula was incorrect as it incorrectly applied the root entropy to the weighted purities of the left and right child nodes. The corrected formula now properly calculates Information Gain by multiplying the root entropy with the weighted sum of the entropies of the left and right child nodes.