Fix "Hello World" Flytekit Tutorial Code

[rachfop]

Description

This PR addresses an issue in the "Hello World" tutorial for Flytekit:

The workflow was named hello_world_wf, but the main execution block tried to run an undeclared my_wf().
This has been corrected to run hello_world_wf().

With this corrections, the tutorial code now runs successfully.

Changes

• Fixed the name mismatch in the main execution block.

Testing

Ensure the tutorial runs without any issues after applying these changes.

python hello_world.py
╭─ Traceback (most recent call ─╮
│ /Users/flytectl/docs/ │
│ hello_world.py:13 in <module> │
│                               │
│ ❱ 13 │   print(f"Running my_w │
╰───────────────────────────────╯
NameError: name 'my_wf' is not