Incorrect equations in Vacamole vignette

[sdwfrost]

The equations for the Vacamole model in the vignette are as follows:

$$dH = \eta I - \gamma H - \eta_2 H - \omega H$$

However, the equations in the header are:

dxdt.col(7) = iToH - hToR - hToD;               // ηI - γH - ωH

[bahadzie]

The equations for the Vacamole model in the vignette are as follows:

$$dH = \eta I - \gamma H - \eta_2 H - \omega H$$

This was an oversight. Earlier in the vignette we dropped the second $H$ compartment. So the equation should be

$$dH = \eta I - \gamma H - \omega H$$

It's been corrected in #273