Fix multi-line latex rendering in vignettes

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.