Change y-axis of transmission spectrum plot on README.md

[nespinoza]

Hi @hover2pi!

This is an extremely minor "bug", but the y-axis on the README.md of the transmission spectrum says "Rp/R_*", whereas what is being plotted is the transit depth. I'm just leaving this as a placeholder as I think this is of extremely low priority, but it might confuse users (I only figured this out after looking at the actual code behind the PLANET_DATA call to hotsoss).

N.