Fix a small inconsistency with Pigeon docs

[skylon07]

What package does this bug report belong to?

pigeon

What target platforms are you seeing this bug on?

Web(site)

Steps to reproduce

1. Go to https://pub.dev/packages/pigeon#rules-for-defining-your-communication-interface
2. Scroll down to #5

Actual/Expected results

This line reads:

5. Generics are supported, but can currently only be used with nullable types (example: List<int?>).

This is no longer true because of #97848. It should just say "Generics are supported" or be removed entirely.

[skylon07]

It felt a little weird filing this as a "bug report", but I didn't know how else to create this issue. I apologize if there's a better place to put it.