docs: change 'hello.py' to 'main.py' in python.md

[my1e5]

A small change to the docs to reflect that hello.py has been renamed to main.py in current versions of uv. See

• Create main.py instead of hello.py in uv init astral-sh/uv#10369

[Copilot]

Pull Request Overview

Updates documentation to reflect that hello.py has been renamed to main.py in current versions of uv, aligning with upstream changes.

• Updates file tree example to show main.py instead of hello.py
• Updates command example from uv run hello.py to uv run main.py

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}