Typo in theoretical_background.Rmd

[adamkucharski]

The theoretical vignette includes the following equations:

$\log S(\infty) = - R_0 [1-S(\infty) ] $

$\mathrm{log}~\phi -R_0 (1-\phi) = 0 = F(\phi) $

But because $S(\infty) = 1-\phi$, I think the latter equation should be

$\mathrm{log}~(1-\phi) + R_0 \phi = 0 = F(\phi) $

to match the model implementation.