doc: correct hidden() to visible() method

[Michael-HEIW]

• the hidden() method was incorrect:

// was:
->hidden(fn (Post $record): bool => auth()->user()->can('update', $record))
// should be:
->hidden(fn (Post $record): bool => ! auth()->user()->can('update', $record))

• I think the visible() method is easier to read, in this example.

// now:
->visible(fn (Post $record): bool => auth()->user()->can('update', $record))

[danharrin]

Thanks!