Also update the related step in the documentation

Accordingly add the version tag to the `image` key in the YAML

tutorials/katacoda/tasks/builders.md

@@ -17,7 +17,7 @@ specs:
       # The builder image to use
       # Here you will use the official Python image from DockerHub (https://hub.docker.com/_/python)
       # which includes the pytest framework
-      image: python
+      image: python:3.9
       # The command to run with the builder image
       # Here you will run the bash shell to start the Python interpreter
 			script: |