Description of serial interval in transmissibility report

[CarmenTamayo]

At the moment the description of the serial interval in the transmissibility report states "Here, we assume that the mean and standard deviation of the $si$ is known, and provided as an input by the user."

However, in reality these parameters can either be provided by the user or taken from the {epiparameter} database.
This will be updated in the tx_pipeline_update PR #144