use the plural version of empty checklist placeholder text

Author: tibbi

app/src/main/res/layout/fragment_checklist.xml

@@ -41,7 +41,7 @@
             android:background="?attr/selectableItemBackground"
             android:gravity="center"
             android:padding="@dimen/activity_margin"
-            android:text="@string/add_new_checklist_item"
+            android:text="@string/add_new_checklist_items"
             android:textSize="@dimen/bigger_text_size"
             android:visibility="gone"/>
     </RelativeLayout>