docs(GlButton): fix broken link to GlLink docs

What does this MR do?

GlButton's docs currently contain a link to https://gitlab.com/gitlab-org/gitlab-ui/blob/master/documentation/link.md, which doesn't exist. This MR replaces that broken link with an URL to GlLink's default story.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Closes #699 (closed)