Improve CSS for Markdown code blocks in Swagger UI v3

[hkosova]

Swagger UI 3.0.10 (as of commit 753134e)
Spec: https://raw.githubusercontent.com/swagger-api/swagger-editor/2.x/spec-files/instagram.yaml

This issue was brought up on Stack Overflow.

Swagger UI v2 displays ```fenced code blocks``` in description like proper code blocks:

Swagger UI v3 displays code blocks like this - not really as blocks of code:

Can we please have better CSS for code blocks in UI v3?