Fix typo in bounding box example

[formigone]

The example in the tf.image.draw_bounding_boxes documentation has a typo that makes it really confusing. The relative values for [y_min, x_min, y_max, x_max] are computed by:

y_min = y_min_px / height
y_max = y_max_px / height
x_min = x_min_px / width
x_max = x_max_px / width

Since the example doesn't explicitly describe this simple formula, the fact that the absolute pixel value for x_max is wrong can make the reader wonder if there is in fact another formula for computing x_max.

[qlzh727]

Thanks for fixing the document.