Tweak packer2019 description

[Munfred]

reviewer comment:

“- packer2019 - 90k cells profiled across multiple timepoints of development.” : I would say “embryonic development” to clarify the relationship of these stages to the other two datasets

we should add the word embryonic:
This is a visualization of the Packer et al 2019 C. elegans single cell RNA sequencing dataset, 89,701 C. elegans embryo cells with across multiple time points of embyonic development