docs: Fixed incorrect theme variable used in theme file (#22761)

* Corrected theme variable from $theme to $my-theme

(cherry picked from commit 2600d4a)

Author: ArcaneAj

guides/theming-your-components.md

@@ -220,5 +220,5 @@ $my-theme: mat.define-light-theme((
 ));
 
 @include mat.all-component-themes($my-theme);
-@include carousel.theme($theme);
+@include carousel.theme($my-theme);
 ```