[Suggestion] Move subsubtopic "Upgrading" in README.md a level up

[btreut]

Is your feature request related to a problem? Please describe.
In early 2024 I started a discussion about missing instructions for upgrading the distrobox installation and found out about a week later that there are instructions for upgrading hidden below alternative installation methods and closed the discussion as solved with a plea for what I repeat now as a suggestion. Since I was a bit frustrated that I did neither remember where the info was hidden nor did I find my discussion immediately since I first searched issues and not discussion.

Describe the solution you'd like
I think everything is expressed in the title, but I repeat it here:

Move subsubtopic "Upgrading" in README.md a level up so that it will show up in the table of contents.

Describe alternatives you've considered
could also be moved moved up more than one level :-)

Additional context
the discussion with myself can be found here: #1135

[btreut]

Grazie mille