chore(pivot): Add showRowHeaders for pivot excel sample (#3472)

Author: dkamburov

src/app/pivot-grid/pivot-export/pivot-export.component.html

@@ -5,6 +5,6 @@
     </div>
 
     <igx-pivot-grid #grid [data]="data" [pivotConfiguration]="pivotConfig" [rowSelection]="'single'"
-        [superCompactMode]="true" [defaultExpandState]='true'>
+        [superCompactMode]="true" [defaultExpandState]='true' [pivotUI]="{ showRowHeaders: true }">
     </igx-pivot-grid>
-</div>
+</div>