Add missing $array variable to It block in AboutArrays Koan - #385

There is a missing $array variable assignment in one of the array koan tests. The test will always fail unless you provide a value for $array.

Noticed while practicing the Koans.

Updates AboutArrays.Koans.ps1

Checklist

• Pull Request has a meaningful title.
• Summarised changes.
• Pull Request is ready to merge & is not WIP.
• Added tests / only testable interactively.
  • Make sure you add a new test if old tests do not effectively test the code changed.
• Added documentation / opened issue to track adding documentation at a later date.