Update 4-personalize-codespace.md and add code to execute the setup.s…

[tfredrick112]

Summary

The postCreateCommand in the devcontainer.json has to be updated so that the setup.sh file gets executed on creation of the codespace. Only after this is done, we can execute the "sl" command on the terminal successfully.

Changes

Adds instructions to update the devcontainer.json file; specifically, the "postCreateCommand" has to be updated.

Closes: https://github.com/skills/code-with-codespaces/issues/138

Task list

• [ ] For workflow changes, I have verified the Actions workflows function as expected.
• [X] For content changes, I have reviewed the style guide.