fix: markup for table caption

[stephiescastle]

Checklist

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Description

Fixes markup of the table caption for accessibility. The previous version was also causing hydration errors.

Instructions to test

1. make vue-storybook
2. view http://localhost:6006/?path=/story/components-blocks-blockrichtable--base-story&globals=theme:ThemeEdu

Tested in the following environments/browsers:

Operating System

• macOS
• iOS
• iPadOS
• Windows

Browser

• Chrome
• Firefox ESR
• Firefox
• Safari
• Edge