Replace git switch with git checkout in docs

[dlachaume]

Content

This PR includes a fix in Mithril documentation.
git switch command was replaced by git checkout in guides to handle checking out tags.

Pre-submit checklist

• Branch
  • Tests are provided (if possible)
  • Crates versions are updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • No clippy warnings in the CI
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested
• Documentation
  • Update README file (if relevant)
  • Update documentation website (if relevant)
  • Add dev blog post (if relevant)

Comments

Commands were updated for both versions (current & next).

Issue(s)

Closes #1174

[jpraynaud]

LGTM 👍