fix: tiny preprocessing bug in complete sc analysis

[ukmrs]

Hello, I loved the tutorial! I found this small bug when playing around with it:

in the pp() when annotating mitochondrial genes: lowercase startswith("mt-") won't catch anything in this data so I just changed it to startswith("MT-")

I'd understand if this isn't a PR worthy change, but I found this confusing when following the tutorial so I thought I would propose a change.