replace parameters in code to reflect relationship with R0

Hi @Degoot-AM 

Given that the $R_0 = \frac{\beta}{\gamma}$

Does it make more sense to write in the parameters $\beta = R_0 \times \gamma$ ?

Thus, instead of

https://github.com/epiverse-trace/tutorials-late/blob/6155ce79d899630c910bf543de6c220480a9678b/episodes/simulating-transmission.Rmd#L347

Better to express it elsewhere as:

```r
transmission_rate <- basic_reproduction * recovery_rate
```