docs(GlCollapse): update docs to use correct buttons

What does this MR do?

A tiny MVC to align the storybook collapse docs with our updated code-base.

Before	After

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Conformity

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• GitLab UI's contributing guidlines.
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  • [-] Customers Portal: mr_url
  • [-] Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.

Security

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
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