Use pims.as_grey instead of image[:, :, 1] in walkthrough tutorial.

[kevinsmia1939]

This tutorial https://soft-matter.github.io/trackpy/v0.6.1/tutorial/walkthrough.html suggest using

def gray(image):
    return image[:, :, 1]  # Take just the green channel

frames = gray(pims.open('../sample_data/bulk_water/*.png'))

To open image as grey scale.

But since pims.as_grey are more concise, should this be change?