Remove border from codeblocks in callouts

[lamATnginx]

Proposed changes

Closes https://github.com/nginxinc/docs-platform/issues/400

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After:

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[nginx-jack]

LGTM - There's some strangeness when the codeblock is part of a list, but we can look at that when we're going through the spacing tickets in general, its not coming from the codeblock itslef.