gef-extras can fail on older Python due to typing error

[CuckooEXE]

list[str] isn't valid in older python versions, it should be List[str]

Description/Motivation/Screenshots

This PR just fixes a small issue in generator.py where list[str] was placed instead of List[str]

How Has This Been Tested ?

N/A

Checklist

• [ x ] My code follows the code style of this project.
• [ x ] My change includes a change to the documentation, if required.
• [ x ] If my change adds new code,
  adequate tests have been added.
• [ x ] I have read and agree to the
  CONTRIBUTING document.