Does declaring victory end your turn?
The offical rules (bottom of page 5) state:


  ENDING THE GAME
  
  If you have 10 or more victory points during your turn the game ends and you are the winner!


Interpreted literally, this is a state of the game, not an action - if at any point during your turn, you have 10 or more victory points, you win, independent of what you say or do. "Declaring victory" is not something which is covered in the rules, so it can't end your turn. However, this literal interpretation of the rules leads to an obvious problem if somebody does have 10 victory points on their turn but doesn't declare victory - are they still the winner?

As such, this is going to depend on the context you're playing in - in any of my gaming groups, we'd give the person claiming victory a ribbing for not being able to count to 10 and let them continue their turn - but that's a decision for your group to make, not one the rules are going to make for you; you can't expect the rules to cover every mistake a player can possibly make. If you're in a more formal context like a tournament, this is the time to call the referee over to your table - that's what they're there for.