[styleguide] update path to scss theme (elastic#140742)

STYLEGUIDE.mdx

@@ -630,7 +630,7 @@ Doing so ensures the styles are never separated or lost on import and allows
 for better modularization (smaller individual plugin asset footprint).
 
 All SASS (.scss) files will automatically build with the [EUI](https://elastic.github.io/eui/#/guidelines/sass)
-& Kibana invisibles (SASS variables, mixins, functions) from the [`globals_[theme].scss` file](src/core/public/core_app/styles/_globals_v8light.scss).
+& Kibana invisibles (SASS variables, mixins, functions) from the [`globals_[theme].scss` file](src/core/public/styles/core_app/_globals_v8light.scss).
 
 While the styles for this component will only be loaded if the component exists on the page,
 the styles **will** be global and so it is recommended to use a three letter prefix on your