A popular Texas myth, but a myth nonetheless. All of the statehood acts are easily found online and no such language can be found. What CAN happen is that Texas can exit the Union the same way it entered, via a bill passed by both houses of Congress and signed by the President. I'd start by electing state representatives who will vote to submit it as a ballot resolution. So far, though, I don't think anyone anywhere has won election on a secession platform since, oh, 1860.
Well, if things keep on going the way they are going, it might become a pretty popular platform somewhere down the line.