Update py_trackbar.markdown

For some mysterious (for me) reason, two lines of the code appears in my browser (Chrome) in the same line.
I've add an "enter" (just pressed enter), but don't know if it's the best solution.

doc/py_tutorials/py_gui/py_trackbar/py_trackbar.markdown

@@ -37,6 +37,7 @@ cv.namedWindow('image')
 
 # create trackbars for color change
 cv.createTrackbar('R','image',0,255,nothing)
+
 cv.createTrackbar('G','image',0,255,nothing)
 cv.createTrackbar('B','image',0,255,nothing)