Docs: Add missing type="button" to Cheatsheet nav buttons

[coliff]

Description

Fix a very minor issue (reported by Webhint):
https://webhint.io/docs/user-guide/hints/hint-button-type/

Motivation & Context

The default type for is submit (not type="button" as one might expect). This can lead to surprising keyboard behavior within a form. We added type="button" to all other buttons on the site - makes sense to add them here too.

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-39585--twbs-bootstrap.netlify.app/

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