Inverting Bistable Multivibrator using OpAmp
Your title is misleading - there is no bistable multi-vibrator in the circuit.

The circuit diagram as drawn has the op-amp shown with positive feedback, in this configuration it is a Schmidt trigger, which would give the output you want.

However, you have constructed the circuit with the +/- inputs (pins 2 and 3) the wrong way round - giving negative feedback. In this configuration the op-amp acts as a linear amplifier with a gain of 2 (1 + Rf/Rg)