docs: quote pip extras install examples

[nyxst4ck]

Summary

• Quote pip install examples that include extras.

Why

• Shells such as zsh can expand unquoted bracket expressions before pip receives them.

Scope

• Files changed:
• phases/03-deep-learning-core/12-intro-to-jax/docs/en.md
• This does not change dependencies or runtime behavior.

Testing

• git diff --check

Maintainer Fit

• Small install-command correctness fix.
• Duplicate PR search terms checked: quote pip extras, quote optional dependency, zsh pip install extras, pip extras install examples

[nyxst4ck]

Quick status note on the Vercel check: the failure is not exposing a build/test error from this branch. GitHub reports:

Vercel: Authorization required to deploy.

The target URL points to Vercel's Git authorization flow for the project/team. I do not have access to authorize that deployment from this fork, so there is no code change I can push here to clear the Vercel preview check. The PR remains draft and mergeable from GitHub's side.