Fixes for FX docs

[aballano]

• Fixed some small errors causing the docs samples not to run properly.
  • effects/fx/README.md -> Removed ank from suspend composition example since there's no main function and the example doesn't need it.
  • effects/fx/async/README.md -> Fixed example in Cancellation by moving the destructuring to a function since "Destructuring declarations are only allowed for local variables/values".
  • effects/fx/polymorphism/README.md ->
    • Fixed one of the examples by converting an expression body main function to block body.
    • Fixed another by adding the k() call for the conversion from List to ListK. Also changed the binding to ! as it's more explicit and less error-prone than destructuring, which also works for lists.

[raulraja]

is this still relevant with latest from master?

[raulraja]

Thanks @aballano!