Update learn.md

not sure it works - but latex is not rendered correctly now for the "where" section - added "$$"

Author: mimi2626

questions/37_calculate-correlation-matrix/learn.md

@@ -10,8 +10,8 @@ $$
 $$
 
 #### Where:
-- \( \text{cov}(X, Y) \) is the covariance between \( X \) and \( Y \).
-- \( \sigma_X \) and \( \sigma_Y \) are the standard deviations of \( X \) and \( Y \), respectively.
+- $$ \text{cov}(X, Y) $$ is the covariance between \( X \) and \( Y \).
+- $$ \sigma_X $$ and $$ \sigma_Y $$ are the standard deviations of \( X \) and \( Y \), respectively.
 
 ### Problem Overview
 In this problem, you will write a function to calculate the correlation matrix for a given dataset. The function will take in a 2D numpy array \( X \) and an optional 2D numpy array \( Y \). If \( Y \) is not provided, the function will calculate the correlation matrix of \( X \) with itself.