Update dependency markdown-it-anchor to v8.1.3

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
markdown-it-anchor	8.1.2 -> 8.1.3

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Release Notes

valeriangalliat/markdown-it-anchor

### [`v8.1.3`](https://togithub.com/valeriangalliat/markdown-it-anchor/blob/master/CHANGELOG.md#​813---2021-08-24) [Compare Source](https://togithub.com/valeriangalliat/markdown-it-anchor/compare/v8.1.2...v8.1.3) - Fix `tabIndex` type. (\[[#​103](https://togithub.com/valeriangalliat/markdown-it-anchor/issues/103)])

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