Fixed doc error in optical flow

doc/py_tutorials/py_video/py_lucas_kanade/py_lucas_kanade.markdown

@@ -46,7 +46,7 @@ get the following equation:
 
 where:
 
-\f[f_x = \frac{\partial f}{\partial x} \; ; \; f_y = \frac{\partial f}{\partial x}\f]\f[u = \frac{dx}{dt} \; ; \; v = \frac{dy}{dt}\f]
+\f[f_x = \frac{\partial f}{\partial x} \; ; \; f_y = \frac{\partial f}{\partial y}\f]\f[u = \frac{dx}{dt} \; ; \; v = \frac{dy}{dt}\f]
 
 Above equation is called Optical Flow equation. In it, we can find \f$f_x\f$ and \f$f_y\f$, they are image
 gradients. Similarly \f$f_t\f$ is the gradient along time. But \f$(u,v)\f$ is unknown. We cannot solve this