Docs: fix dark offcanvas dismiss attribute

[Kevinjohn]

Description

Fixes the dark offcanvas example's close button dismiss attribute.

Motivation & Context

The copied dark offcanvas example used data-bs-dismiss="offcanvasDark", but Bootstrap's dismiss trigger listens for data-bs-dismiss="offcanvas". When the example is used in a live offcanvas, clicking the close button does not dismiss the component.

Expected: clicking the close button dismisses the offcanvas.
Actual: the offcanvas remains shown because the dismiss selector does not match.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-42706--twbs-bootstrap.netlify.app/

Related issues

None.

Checks

• git diff --check
• npm exec -- prettier --config site/.prettierrc.json -c site/src/content/docs/components/offcanvas.mdx
• npm run docs-build