fix an error in the formula for cv::cornerSubPix

modules/imgproc/include/opencv2/imgproc.hpp

@@ -1853,7 +1853,7 @@ where \f${DI_{p_i}}\f$ is an image gradient at one of the points \f$p_i\f$ in a
 value of \f$q\f$ is to be found so that \f$\epsilon_i\f$ is minimized. A system of equations may be set up
 with \f$\epsilon_i\f$ set to zero:
 
-\f[\sum _i(DI_{p_i}  \cdot {DI_{p_i}}^T) -  \sum _i(DI_{p_i}  \cdot {DI_{p_i}}^T  \cdot p_i)\f]
+\f[\sum _i(DI_{p_i}  \cdot {DI_{p_i}}^T) \cdot q -  \sum _i(DI_{p_i}  \cdot {DI_{p_i}}^T  \cdot p_i)\f]
 
 where the gradients are summed within a neighborhood ("search window") of \f$q\f$ . Calling the first
 gradient term \f$G\f$ and the second gradient term \f$b\f$ gives: