Updated callout example

I updated the callout example to use a primary
button rather than a secondary button, to
provide better contrast.

I don't think this is a pattern we want to
inadvertantly encourage via our docs.

packages/eui/src-docs/src/views/call_out/warning.tsx

@@ -8,7 +8,7 @@ export default () => (
  Here be dragons. Don’t wanna mess with no dragons. And{' '}
  <EuiLink href="#">here&rsquo;s a link</EuiLink>.
  </p>
- <EuiButton href="#" color="warning">
+ <EuiButton href="#" color="warning" fill>
  Link button
  </EuiButton>
  </EuiCallOut>