What is the ground state molecular term symbol for $\ce{HF}$?

My attempt went like this: I drew the molecular orbital diagram and found that in the ground state, the highest energy MOs were two degenerate orbitals with 2 electrons each. Since there are no unpaired electrons, the term symbol assigned should convey the the information: (1) $\Lambda = 0$, (2) Spin multiplicity of 1, and (3) symmetric or anti-symmetric about a defined mirror plane containing the internuclear axis.

With information (1) and (2), the term symbol I obtained is $^1\Sigma$. But, I am not entirely sure about information (3), should it be $^1\Sigma^+$ or $^1\Sigma^-$?