Fix typo in slim-select_default.md

[dlackty]

Motivation / Background

This Pull Request has been created because there's a typo in README sample codes.

Detail

This Pull Request changes a typo in README sample codes.

Additional information

Checklist

Before submitting the PR make sure the following are checked:

• [x] This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• [x] Commit message has a concise description of what changed and why.
• [ ] Tests are added or updated if you fix a bug or add a feature.
• [ ] Documentation has been added or updated if you add a feature or modify an existing one.
• [ ] CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature (under the "Unreleased" heading if this is not a version change).
• [x] My changes don't introduce any linter rule violations.