fix(material/theming): fix not found error in the guide duplicate the…

…ming styles document (#30108)

point to the v18 version of the document
resolves #30072

src/material/core/theming/_theming.scss

@@ -15,7 +15,7 @@ $_generate-default-density: true !default;
 
 // Warning that will be printed if duplicated styles are generated by a theme.
 $_duplicate-warning: 'Read more about how style duplication can be avoided in a dedicated ' +
-  'guide. https://github.com/angular/components/blob/main/guides/duplicate-theming-styles.md';
+  'guide. https://v18.material.angular.io/guide/duplicate-theming-styles';
 
 // Warning that will be printed if the legacy theming API is used.
 $private-legacy-theme-warning: 'Angular Material themes should be created from a map containing ' +