The measurement of the extra sensor, measuring through which slit the electron goes, ‘collapses the wavefunction’ of the electron to only two possible states: the electron either goes through slit one or slit two. The intensity pattern is therefore just two bands according to the slits.
We can also see this from the sketch of the double slit experiment in the slides. By measuring through which slit each electron goes, we know that there is only one probability amplitude per electron that will contribute to the intensity: only the one from the slit where the electron went through. This eliminates the interference term and we only have two bands on the screen.