Update astropy pixel count figure

[andrewgait]

The figure on this line https://github.com/UoMResearchIT/programming_with_python/blob/039da4b3c37b2833058fa3f709517650de8144c7/episodes/03-numpy_essential.md?plain=1#L420 does not quite look the same when I reproduced the code just now:

The formatting of the notes is also not quite right - I think there needs to be a blank line in between the figure and the next sentence.

[andrewgait]

The same is true for other histogram figures in this section, so they could also be updated in this ticket.

[andrewgait]

Looking at this more closely, the only difference is the extra information that Jupyter itself is printing above the figures. This is not actually part of the figure itself, so I don't think it's worth going through and updating to add this - I'm just going use this ticket to sort the formatting out.