fix wrong method name

doc/py_tutorials/py_gui/py_trackbar/py_trackbar.markdown

@@ -14,7 +14,7 @@ Here we will create a simple application which shows the color you specify. You
 shows the color and three trackbars to specify each of B,G,R colors. You slide the trackbar and
 correspondingly window color changes. By default, initial color will be set to Black.
 
-For cv.getTrackbarPos() function, first argument is the trackbar name, second one is the window
+For cv.createTrackbar() function, first argument is the trackbar name, second one is the window
 name to which it is attached, third argument is the default value, fourth one is the maximum value
 and fifth one is the callback function which is executed every time trackbar value changes. The
 callback function always has a default argument which is the trackbar position. In our case,