Fixed the code for the spectral function calculation

[mesonepigreco]

The equation for the spectral function was changed from
$$
\sigma(\omega) = - \frac{\omega}{\pi}\Im G(\omega + i\eta)
$$
Into
$$
\sigma(\omega) = - \frac{1}{\pi}\Im\left[ (\omega + i\eta) G(\omega + i\eta)\right]
$$
This ensures the sum rule's validity as $\omega\to0$ even for finite values of the smearing $\eta$.