Correct doc string in the concentration method

The integrated_concentration methods return the exposure which is the number of viruses per m3 of room volume (virions/m3) and not the dose which is in infectious virions.

when integrated C(t) dt you get exposure * t which is in virions * h / m3 then, when multiplied by the breathing rate (m3/h) you get the dose in virions.

The math is correct (I assume?), just the docstring caused confusion when performing the plots for the paper.