Fix syntax error in W&B weave notebook

[zanieb]

Summary

This is a very minor change which updates the W&B Weave example notebook to use valid syntax.

Motivation

For context, there's a variable which they want the user to assign a value to e.g. FOO = # set a value but this is invalid syntax and could be confusing to some users. I've edited it to be FOO = "" # set a value. I discovered this while attempting to include this project in Ruff's ecosystem checks at astral-sh/ruff#9293.

There is a minor chance that this would increase confusion as the notebook will no longer fail with a syntax error if the user does not set a value. It'd be trivial to add additional code that checks the value of the variable and raises an informative error if that's a concern. I am happy to do so, but wanted to start with the smaller changer.

[shyamal-anadkat]

thank you!