Sure: "without reference to the citizenship of their parents."
And, obviously, if the citizenship of the parentS presents no barrier, then even less of a barrier is presented by the case when one parent is a citizen, and the other is not.
And given that this is the scenario under which Obama's citizenship must be considered.... "parent or parents" is the appropriate formulation.
....it was never doubted that all children born in a country of parents who were its citizens became themselves, upon their birth, citizens also.
Dude......