fix: corrected stepper header style to apply only for accordion header

Author: thiagu

src/helix-ui/styles/modules/stepper.less

@@ -11,7 +11,7 @@ hx-accordion.beta-hxStepper {
       border-bottom: 1px solid @gray-300;
     }
 
-    header[slot="header"] {
+    > header[slot="header"] {
       display: flex;
       padding: 1rem;