Update ListView control iconFieldName to reflect a property that does show Icon

I was having issues showing the iconFieldName and eventually found my answer in this closed issue:
#396 (comment)

But it does not seem like the docs were ever updated so thought I'd spare the next person the same trouble :)

iconFieldName="FileRef" works as of Christmas Day, 2022

Author: mikezimm

docs/documentation/docs/controls/ListView.md

@@ -27,7 +27,7 @@ import { ListView, IViewField, SelectionMode, GroupOrder, IGrouping } from "@pnp
 <ListView
   items={items}
   viewFields={viewFields}
-  iconFieldName="ServerRelativeUrl"
+  iconFieldName="FileRef"
   compact={true}
   selectionMode={SelectionMode.multiple}
   selection={this._getSelection}