message: fix an error in the formula for cv::cornerSubPix
diff: @@ -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:
  