Fix comment in ATExecValidateConstraint

The comment seems to have been copied from ATExecAddColumn, which says:

 /*
  * If we are told not to recurse, there had better not be any
- * child tables; else the addition would put them out of step.

For ATExecValidateConstraint, it should say something like:

+ * child tables; else validating the constraint would put them
+ * out of step.

Attached patch fixes it.

Thanks,
Amit