Fix `unless` code comment

[fritz-c]

The existing code comment was copied from when, and therefore mistakenly explains the callback is called when the value is truthy.

And just for reference, here was my own sanity check, done in tinker:

> collect([1,2,3,4])->unless(false,fn($a)=>$a->prepend(62362))
= Illuminate\Support\Collection {#7422
    all: [
      62362,
      1,
      2,
      3,
      4,
    ],
  }

> collect([1,2,3,4])->unless(true,fn($a)=>$a->prepend(62362))
= Illuminate\Support\Collection {#7351
    all: [
      1,
      2,
      3,
      4,
    ],
  }