docs: fix `$effect` section examples

[navorite]

It seems #12200 is back again after the site migration, maybe it is worth checking if previous docs fixes exist or not.

closes #13912

Before submitting the PR, please make sure you do the following

• [ ] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• [ ] Prefix your PR title with feat:, fix:, chore:, or docs:.
• [ ] This message body should clearly illustrate what problems it solves.
• [ ] Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• [ ] Run the tests with pnpm test and lint the project with pnpm lint