fix: correct reversed types in 03-split-on

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: [type-operations/template-literal-type-shenanigans/03-split-on] Incorrect logic in solution #283
• That issue was marked as accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Tested with:

type Test1 = SplitOn<"baby", "a">;
//   ^? type Test1 = ["b", "by"]

type Test2 = SplitOn<"hello my baby", " ">;
//   ^? type Test2 = ["hello", "my", "baby"]

💖