tauRecTools: use the right equation to calculate combined sigma
We combine the four momentum of two calibrations in CombinedP4FromRecoTaus. The problem is that the calculation is not right for the combined sigma. It should be sigma(combined) = sigma_1^2 * sigma_2^2 * (1 - rho^2) / (sigma_1^2 + sigma_2^2 - 2 * pho * sigma_1 * sigma_2)