docs/extensions/tables.md: Fix header formatting

[mitya57]

Description

Currently the header is rendering as:

<p>&ndash;
title: Tables Extension</p>

which can be seen here.

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Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
• The commit message follows the Commit Message Style Guide.
• I have added or updated relevant docs, including release notes if applicable which follow the Documentation Style Guide.
• I have added or updated relevant tests.
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