Really, no worries. This is one of the most common warnings we give out here. I actually found another one moments after yours! It's not even close to being a big deal as long as it gets fixed.
Warnings are just ways of notifying a user about inappropriate conduct so they can fix it quickly, it doesn't have an effect on anything. If a problem persists we give out infractions, and the warning serves to remind the other moderators that a user did the same thing before. But it's really not anything to worry about because a signature mistake is the most harmless things you can do here. We only get concerned if it's something like foul language or harassment. So yeah, you're fine. ^^