Update screenshots in download section to reflect name change of libraries_metadata.tsv to single_cell_metadata.tsv

[allyhawkins]

Related to changes in #56, we have changed the name of the libraries_metadata.csv that is included in the download from scpca to be single_cell_metadata.tsv. This change should be reflected in the screenshots for downloading by project and downloading by sample as that file is present in both.

The PR with the screenshot changes should be stacked on #56 so that we can merge all changes at the same time and not have conflicting information available in the docs.

[jaclyn-taroni]

These are not screenshots, they are illustrations. cc @dvenprasad We'll need these before we launch.

[dvenprasad]

[allyhawkins]

Closed by #58