Update prepdocs.sh to use python3

[pamelafox]

Purpose

It's generally safer to use python3 than python since the latter is an alias (and does not work on my Mac). python3 should exist, however, if Python 3 is involved.

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Run azd up on Mac
• Don't see error about python: command not found