fix equation terms

[avallecam]

in the first eq, to match with eq6 from Nishiura et al., 2009.

in the second eq, the y term was not clear to me, so we could replace it with the corresponding terms

[pratikunterwegs]

Thanks Andree, I'll take a look in a bit, but looks good to me initially.

[pratikunterwegs]

Thanks @avallecam - this looks like it makes sense.

For future development reference, the first equation is now in line with Nishiura et al.'s Eqn. 6 for the factor of under-ascertainment.