Improve documentation terminology for ROI in normalize_roi

[KedoKudo]

Is your feature request related to a problem? Please describe.
The current documentation of normalize_roi is confusing as it refers an integer as pixel coordinates, which should be a tuple for an 2D image.

Describe the solution you would like.
It is better to provide an ASCII art to show case what these four integers actually refers to.

Describe alternatives you've considered.
The alternative would be change the inputs into actual pixel coordinates as a list of [(row, col),... ], however this API change would probably break a lot of downstream code depending on Tomopy.

Additional context.
N/A

[KedoKudo]

@JeanBilheux

[carterbox]

tomopy/source/tomopy/prep/normalize.py

Line 145 in f26ae62

def normalize_roi(arr, roi=[0, 0, 10, 10], ncore=None):