No, it's not incorrect. It's just a partial quote. Ideally, they should end it with ellipses (...) to indicate that it is just a partial quote. But it's not very unusual or unprecedented for famous sayings to be quoted partially with a full stop.
Partial quotes are made to highlight a particular phrase in support of a particular point, without the distraction of the rest of the text which is irrelevant to the point being made. The source of the quote is given so a reader can easily look it up and see the whole text.
My guess is that they quoted Matthew 24:14 partially just to make the point that [they believe] they're fulfilling the prophecy about the good news being preached. The rest of the text is irrelevant to that point. If the full quote were given then it would be less clear exactly which point they're making - that the end is coming; or that they're fulfilling the preaching of the good news of the kingdom; or both? Since they only wanted to make the clear point that they're preaching the good news they quoted only that part. IMHO it's a fair use of partial quotes.
Watchtower does engage in misleading partial quotes where inconvenient information directly related to the point they're making is omitted, but I don't think this is one of them.