An initiatory petition for the amendment of this charter shall state the proposed amendment, shall set forth all sections of the charter to be amended thereby in full, shall be signed by the number of electors required for the initiation of code provisions, and shall be filed with the Mayor. If the petition therefor is proper and in order, the Mayor shall submit the proposed amendment to the electors of the city at the next regular city election which is to be held after ninety days following the filing of the petition. Amendment of the charter shall become effective if approved by a majority of qualified voters voting on the question.