jupyterhub_for_teaching: Use correct image

assigned to @mflehmig--tu-dresden.de

@mflehmig--tu-dresden.de I took the image from Compendium_attachments/JupyterHubForTeaching/url-git-pull-and-quick-start.png if this isn't used anywhere else, I can remove it.

requested review from @rotscher--tu-dresden.de

approved this merge request

merged

mentioned in commit a557bdcd

Sieht gut aus.

Danke Dir @wesarg--tu-dresden.de