Update LLM attr tutorial notebook with note on required Captum version (#1496)

Summary:
Pull Request resolved: #1496

Got #1486 when I was oncall a while ago. A user ran into this error:
> TypeError: ShapleyValues.attribute() got an unexpected keyword argument 'skip_tokens'

due to being on a package of Captum where
> LLMAttribution.attribute() doesn't have skip_tokens as a parameter, and therefore it's being passed to ShapleyValues.attribute() as part of **kwarg, though ShapleyValues never supported skip_tokens

It's not sufficient to be on the latest Captum release (v0.7), atp they need to install from Github

Reviewed By: craymichael

Differential Revision: D69566114

fbshipit-source-id: 38f5fdc46c610a8ab1e8a21ef375121fa18c9606

Author: sarahtranfb