Add explicit inline link to Barnum et al. 2018

[taylorreiter]

In lesson 08_bin_completion_with_spacegraphcats.md, we use a figure from Barnum et al 2018 depicting pcrA. The image source is attributed in the image link, but this does not appear inline in the text and is difficult to find.

The image link looks like this:

![source: ISME J 12, 1568–1581 (2018) doi:10.1038/s41396-018-0081-5](_static/41396_2018_81_Fig4_HTML.png)

The attribution should be either https://www.nature.com/articles/s41396-018-0081-5 or the doi (doi:10.1038/s41396-018-0081-5) or both, and should occur in the paragraph above the image.