Any city which has adopted the council-manager plan, as herein provided, and which shall have operated under the council-manager plan for a period of three years, may discontinue the council-manager plan of government by a vote of the electors thereof initiated by a petition of twenty per centum of the qualified electors of said city, and the same procedure set forth in Section 21-9-3, shall be followed in determining whether such plan shall be discontinued. The propositions to be then voted on shall be: "For the council-manager form of government" and "Against the council-manager form of government."