fix mistakes about @import supports() syntax

[Erithax]

The previous css example included invalid syntax (see spec) and the explanation below was confusing.

[github-actions]

Preview URLs

• /en-US/docs/Web/CSS/@import

Flaws (2)

URL: /en-US/docs/Web/CSS/@import
Title: @import
Flaw count: 2

• macros:
  • Wrong xref macro used (consider changing which macro you use). Error processing path /en-US/docs/Learn//CSS
  • Wrong xref macro used (consider changing which macro you use). Error processing path /en-US/docs/Learn//Getting_started_with_the_web/CSS_basics