fix formula name

Author: komaksym

old_repo/Problems/138_gini_impurity/learn.md

@@ -26,7 +26,7 @@ A pure node (all one class) has \( G = 0 \), and higher values indicate more cla
 
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-## Gini Gain for a Split
+## Weighted Gini Impurity
 
 Given a feature and a threshold to split the dataset into left and right subsets: