Guides CSS to format code tags in h2 headings [ci skip]

[andycroll]

Motivation / Background

Noticed when reading the edge guides that we have some wonky formatting for the case when we use a method name in an h2 heading.

There's already a fix for h3-level headings, so I copied the font-weight (400) and eyeballed a font-size. 2.25rem looked too much to my eyes, so went with 2rem.

Detail

This Pull Request changes CSS for the guides. To add h2 code formatting to bring the visual size into line.

Additional information

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Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[p8]

Thanks @andycroll !