Buttons guidance: change examples to conform to Carbon capitalization guidelines

[ericradz]

Can the guidance for Buttons be updated so that all examples that include a multi-word label use sentence-style capitalization? For example, https://react.carbondesignsystem.com/?path=/docs/components-button--overview#overview contains examples that use Tertiary Danger Button, Ghost Danger Button, Danger Tertiary, and Danger Ghost. As per the Capitalization guideline, these should all use sentence-style caps.