[core] docs: remove unnecessary </code> markup

[kswchoo]

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

Removed unnecessary </code> from core typography documentation.

Reviewers should focus on:

Please verify example code in the documentation is still valid.

Screenshot

[palantirtech]

Thanks for your interest in palantir/blueprint, @kswchoo! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[adidahiya]

thanks!