chapters/compute: Move `mini-shell` task from lab 6 to 7

[teodutu]

Prerequisite Checklist

• [x] Read the contribution guidelines regarding submitting new changes to the project;
• [x] Tested your changes against relevant architectures and platforms;
• [x] Updated relevant documentation (if needed).

Description of changes

The task requires knowledge of fork() and execve(), the latter being taught in lab 7.

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Published at https://cs-pub-ro.github.io/operating-systems/136/