Add assertion function example

<!-- 👋 Hi, thanks for sending a PR to LearningTypescript! 💖. Please fill out all fields below and make sure each item is true and [x] checked. Otherwise we may not be able to review your PR. --> ## PR Checklist - [ ] Addresses an existing open issue: fixes #000 - [ ] That issue was marked as [accepting prs](https://github.com/LearningTypeScript/site/issues?q=is%3Aopen+is%3Aissue+label%3A%22accepting+prs%22) - [ ] Steps in [CONTRIBUTING.md](https://github.com/LearningTypeScript/site/blob/main/.github/CONTRIBUTING.md) were taken ## Overview Add an actual assertion function example (the one you had wasn't a [typescript assertion function](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions))