SIR typo fix 2

[MichaelKlots]

Same as before - a typo in the SIR model description text

The SIR model formula text in examples/deterministic/ex01a-Model.ipynb has a typo in the dI/dt equation. It was defined there:
$$
\lambda(t) = \frac{\beta I}{N}
$$
and then:
$$
\dot I &= \lambda(t)I - \gamma I
$$

It should be:
$$
\dot I &= \lambda(t)S - \gamma I
$$

as \lambda(t) already has I as a multiplicative factor, and it is \lambda*S in the dS/dt equation. This way the time derivatives would sum up to 0.

[rajeshrinet]

These changes fix typos in description of the model in example notebooks.