Docs: fix nested kbd markup in Reboot example

[monsonego]

Description

• replace the nested <kbd> markup in the Reboot keyboard input example
• render Ctrl and , as separate keyboard entries joined by plain text

Motivation & Context

Issue #38000 points out that the current example marks Ctrl + , as one combined keyboard key because the markup nests multiple <kbd> elements inside another <kbd>.

This change keeps the example visually the same while making the markup semantically correct and easier to understand.

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-42277--twbs-bootstrap.netlify.app/docs/5.3/content/reboot/#user-input

Validation

• ran npm run docs-prettier-check
• reviewed the rendered example source in site/src/content/docs/content/reboot.mdx to confirm the shortcut now uses separate <kbd> elements
• confirmed the Netlify deploy preview was created successfully
• not run the full docs build locally in this environment

Related issues

Closes #38000

[mdo]

Pushed a fix to v6-dev.