Subtle inconsistency in collection

I've noticed when going through the course that the majority of the time `sentinel-2-l2a` is used, except (I think) in a couple of places.

I noticed this because in the final episode "Data cubes with ODC-STAC", `sentinel-2-c1-l2a` is used instead (note the `c1`). I was interested in reproducing the burned area figure produced at the end of episode 9 for the whole island but the metric introduced in episode 9 doesn't work with the reprocessed data.

`sentinel-2-l2a` | `sentinel-2-c1-l2a`
:---------------: | :------------------:
 | 

In the end this isn't a massive problem, since all the material is consistent, and it might be that no one else tries this, but it took me a while to figure out why it wasn't working.