removing "scale" argument in plotting RBG with image stretch

[sjgraves]

Episode 5 (working with multi-band rasters) introduces the concept of image stretching when plotting a full color RGB. After the explanation of image stretching, the plotRGB function is used again, with the added arguments of stretch and scale. For example:

plotRGB(RGB_stack_HARV,
r = 1, g = 2, b = 3,
scale = 800,
stretch = "lin")

It's not apparent that the scale argument is necessary - removing it or changing the value produces the same result. In addition, the value of 800 doesn't make sense, since this argument defines the max value of the 3 channels (which is 255) and defaults to either 255 or the maximum value.

Without an explanation of the scale argument and why it's needed, it seems like it can be removed to keep the function simple.

[jsta]

Sounds good to me. Could you open a pull request for this change?