Fix definition of the covariance matrix of derivatives.

modules/imgproc/doc/feature_detection.rst

@@ -62,7 +62,7 @@ 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.