decrease the font size of response preview headers

[e-maud]

It's really a not urgent cosmetic detail, but I write it before I forget. The API response previews are great, but the headers (e.g., 'Article result', 'FindNewspapers result') are rendered larger than H3, which make them easily confused with markdown headers and disrupting the notebook's structure. When working in a personal notebook with the API, I would prefer slightly smaller font sizes for these headers, allowing the notebook's main structure to stand out more clearly.