Outdated 'enter.md' is referenced in 'shells_in_shells.md'

[GrabbenD]

PR #1587 deleted commands/docs/enter.md but it's still referenced in book/shells_in_shells.md.

What's the new equivalent of this command?

Nu offers the concept of "shells". As the name implies, they're a way of running multiple shells in one, allowing you to quickly jump between working directories and more.

[fdncred]

The docs should be updated. Thanks. We've moved this functionality to the nushell stdlib. In order to use it you must do this use std/dirs shells-aliases *

[NotTheDr01ds]

Thanks for the reminder on this!