correct covariation matrix formula in Harris corner detector

modules/imgproc/doc/feature_detection.rst

@@ -66,7 +66,7 @@ For every pixel
 
 .. math::
 
-    M =  \begin{bmatrix} \sum _{S(p)}(dI/dx)^2 &  \sum _{S(p)}(dI/dx dI/dy)^2  \\ \sum _{S(p)}(dI/dx dI/dy)^2 &  \sum _{S(p)}(dI/dy)^2 \end{bmatrix}
+    M =  \begin{bmatrix} \sum _{S(p)}(dI/dx)^2 &  \sum _{S(p)}(dI/dx dI/dy)  \\ \sum _{S(p)}(dI/dx dI/dy) &  \sum _{S(p)}(dI/dy)^2 \end{bmatrix}
 
 where the derivatives are computed using the
 :ocv:func:`Sobel` operator.