docs: Fixed typo in Implementing In-app Billing guide.

   ArrayList<String>  purchaseDataList =

      ownedItems.getStringArrayList("INAPP_PURCHASE_DATA_LIST");

   ArrayList<String>  signatureList =

      ownedItems.getStringArrayList("INAPP_DATA_SIGNATURE");

      ownedItems.getStringArrayList("INAPP_DATA_SIGNATURE_LIST");

   String continuationToken =

      ownedItems.getString("INAPP_CONTINUATION_TOKEN");