FIX - Multiselect slot schema

headercheckboxicon schema does not corresponds to the actual object
return by slotProps.

`allSelected -> checked`

```ts
<MultiSelect>
  <template #headercheckboxicon="slotProps">
    {{ console.log(slotProps.checked) }}
  </template>
</MultiSelect>
```

Author: maxime_tamburrini

packages/primevue/src/multiselect/MultiSelect.d.ts

@@ -787,7 +787,7 @@ export interface MultiSelectSlots {
         /**
          * Options of the loader items for virtualscroller
          */
-        allSelected: boolean;
+        checked: boolean;
         /**
          * Style class of the loading icon.
          */