• Have you followed the guidelines for contributing?
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• Have you successfully run make lint && make test?

I ran make lint and received errors on files this PR doesn't touch.

Updates to the tutorials based on feedback in UX sessions:

1. When copying the skopeo command for the chiseled hello world rock. The user missed the hint to change the name in the project file to 0.1-chisel, resulting in a error.
2. File headers could be added to file excerpts (app.py) External example
3. Clarification may be needed on what the skopeo command is doing. Maybe we could also mention about how Docker requires rocks (or images in general) to be imported into the daemon since they cannot be executed directly like an executable.

In addition, two corrections to the Express and Spring Boot tutorials:

1. Add mkdir <app rock name> and cd <app rock name> to the beginning of the tutorial so that the rock will have the correct name from the start.
2. Update the port to the correct value.