Update hooks.md

[schlich]

Running the example for environment activation as-is gave the following type mismatch error:

Error: nu::shell::type_mismatch

  × Type mismatch during operation.
    ╭─[/Users/foo/Library/Application Support/nushell/config.nu:27:33]
 26 │                         ('/Users/foo/dir' not-in $after
 27 │                             and '/Users/foo/dir' in $before
    ·                                 ─────────────────────┬──────────────────── ─┬ ───┬───
    ·                                                      │                      │    ╰── nothing
    ·                                                      │                      ╰── type mismatch for operator
    ·                                                      ╰── string
 28 │                             and 'activate' in (overlay list))
    ╰────


~ 
❯ 𝒾: 
process finished with exit code = -1

[fdncred]

This looks right to me because on startup before is null. I rely on that in one env change hook so that the hook only runs at startup. Thanks!