That number form is usually from the State of California. Follow the directions, but it might be addressing the issue about the second W-2.
If the letter IS from the IRS, follow the instructions, you may not need to file an emended 1040X if the IRS can adjust your return.
If the letter is from California, and/or you need to amend your return for the additional W-2, follow the directions below. Be sure to file an amended return for both the Federal and the State, Amended returns need to be printed and mailed.