[BUG] Links to demo datasets for Annotate are dud

[sidneymbell]

Describe the bug
We link to demo datasets to use with desktop explorer from the quickstart docs. URLs all look like https://cellxgene-example-data.czi.technology/FOO-BAR-DATASET.h5ad and they all yield a 404.

[amomin-bmea]

I am getting a similar error message while trying to use cellxgene locally. Can you please look into this bug.

cellxgene launch https://cellxgene-example-data.czi.technology/pbmc3k.h5ad [cellxgene] Starting the CLI... Error: Dataset does not exist.