fix: use fg instead of fg-fg

Author: cylewaitforit

app/design-system-guidelines.mdx

@@ -61,7 +61,7 @@ Only avatars should have fully circular shapes.
 
 ## Colors
 
-- Primary surfaces: **white** (`fg-fg`)
+- Primary surfaces: **white** (`fg`)
 - Secondary or less important content: use softer, muted tones
 
 [See Colors](./?path=/docs/tokens-colors--docs)