Fix incorrect id name in ListActivity's documentation.

 * <p>

 * Optionally, your custom view can contain another view object of any type to

 * display when the list view is empty. This "empty list" notifier must have an

 * id "android:empty". Note that when an empty view is present, the list view

 * id "android:id/empty". Note that when an empty view is present, the list view

 * will be hidden when there is no data to display.

 * <p>

 * The following code demonstrates an (ugly) custom screen layout. It has a list

 *         android:paddingLeft="8dp"

 *         android:paddingRight="8dp">

 *

 *     <ListView android:id="@id/android:list"

 *     <ListView android:id="@android:id/list"

 *               android:layout_width="match_parent"

 *               android:layout_height="match_parent"

 *               android:background="#00FF00"

 *               android:layout_weight="1"

 *               android:drawSelectorOnTop="false"/>

 *

 *     <TextView android:id="@id/android:empty"

 *     <TextView android:id="@android:id/empty"

 *               android:layout_width="match_parent"

 *               android:layout_height="match_parent"

 *               android:background="#FF0000"