Escape @use syntax in docs to avoid clash with docs @tag

Author: ggdouglas

packages/core/src/docs/classes.md

@@ -81,7 +81,7 @@ Blueprint v6 uses the **`bp6-`** prefix for all CSS classes. For example, Button
 When writing custom styles that target Blueprint components, you should reference the namespace using the `$ns` Sass variable rather than hardcoding the prefix:
 
 ```scss
-@use "@blueprintjs/core/lib/scss/variables.scss" as bp;
+\@use "@blueprintjs/core/lib/scss/variables.scss" as bp;
 
 .my-class .#{bp.$ns}-button {
     // Custom styles for Blueprint buttons within .my-class