fix workflow function name

[stolarczyk]

The first code snippet in the "Environment setup" user guide calls and undefined function:

from flytekit import task, workflow

@task
def say_hello() -> str:
    return "Hello, World!"

@workflow
def hello_world_wf() -> str:
    res = say_hello()
    return res

if __name__ == "__main__":
    print(f"Running my_wf() {my_wf()}")

> python hello_world.py

╭─────────────────────────────────────────────────────────────────────────── Traceback (most recent call last) ───────────────────────────────────────────────────────────────────────────╮
│ <path>/hello_world.py:16 in <module>                                                                                                                         │
│                                                                                                                                                                                         │
│ ❱ 16 │   print(f"Running my_wf() {my_wf()}")                                                                                                                                            │
╰─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯
NameError: name 'my_wf' is not defined

This PR fixes the above issue.