chore: fix stylelint warning in button toggle demo (#13345)

Fixes a stylelint failure that was introduced by a PR which was submitted before the line breaks rule was introduced (#12443).

Author: crisbeto

src/demo-app/button-toggle/button-toggle-demo.scss

@@ -1,3 +1,3 @@
-section {
-  margin-bottom: 16px;
-}
+section {
+  margin-bottom: 16px;
+}