Small hello-clusters notebook cleanups

[sjspielman]

Noticed a couple bits to clean up/remove from the first hello-clusters notebook, and I updated the HTML. A couple EOL spaces got eaten along the way, so turn off whitespace if you want (though not really necessary here I think).

• The first removed sentence is no longer accurate
• The TODO probably doesn't make sense given that that's not what that module actually does! But, I opened Consider using a converted Seurat object in hello-clusters #957 in case we want to update this part in the future (I'm on the fence if we should, but that issue will remind me to get off the fence one way or another!)

[jashapiro]

LGTM