Replace removed `runIdentity` with `unwrap`

[fix-fix]

In Monad Transformer Stacks section in chapter 11 REPL example uses runIdentity from purescript-identity package. But this function was removed in v2.0 release and it's advised to use more generic unwrap from purescript-newtype, therefore provided piece of code doesn't work.

The section bits, mentioning runIdentity, should be reworded to use unwrap or other more suitable function.