Adapted URLs again in Quickstart.ipynb

Quickstart.ipynb in example-profile was not working anymore in basic "Python"-Profile.

Dockerfile

 # Specify parent image. Please select a fixed tag here.
-ARG BASE_IMAGE=registry.git.rwth-aachen.de/jupyter/profiles/rwth-courses:2020-ss.1
+ARG BASE_IMAGE=registry.git.rwth-aachen.de/jupyter/profiles/rwth-courses:latest
 FROM ${BASE_IMAGE}
 
 # Install packages via requirements.txt