rustdoc: `code` block within portability span is unreadable on dark theme

[ehuss]

The portability span uses code blocks for target features, but these have a default dark background color that is unreadable on the dark theme.

Sample page: https://doc.rust-lang.org/nightly/core/arch/x86_64/index.html

[GuillaumeGomez]

Indeed, will send a fix.