fix(help): Don't dim placeholders

[epage]

This came from feedback: https://rust-lang.zulipchat.com/#narrow/stream/220302-wg-cli/topic/Help.20changes.20in.20clap.20v4

[joshtriplett]

Thank you.

To elaborate on this: some people set their terminals to white-on-black rather than gray-on-black for readability, and dimmed text can be harder to read.