Fix multi-line latex rendering in vignettes

[adamkucharski]

The below latex syntax is not currently rendering:

$$
E(\text{Total deaths}) = p \sum_{t} \sum_j f_{j} I_{t-j} \\ 
E(p) = \frac{\text{Total deaths}}{\sum_{t} \sum_j f_{j} I_{t-j} }
$$

For now, we use

$$
E(\text{Total deaths}) = p \sum_{t} \sum_j f_{j} I_{t-j} 
$$
$$
E(p) = \frac{\text{Total deaths}}{\sum_{t} \sum_j f_{j} I_{t-j} }
$$

But there should be a more elegant solution.