Vignette: Minor corrections

[LGraz]

Thanks for the package <3

In the vignette Equation 2.1 has a typo and could be improved. The current one is:
$\hat{S}{K M}(t)=\prod{s<t} \frac{\bar{Y}(t s)-d(s)}{\bar{Y}(s)}$

this should be corrected to
$$\hat{S}{K M}(t)=\prod{s<t} \frac{\bar{Y}(s)-d(s)}{\bar{Y}(s)}$$

but now one should still explain what $s$ means (only valid survival times)

I would recommend:
$$\hat{S}{K M}(t)=\prod{k: t_k<t} \frac{\bar{Y}(t_k)-d(t_k)}{\bar{Y}(t_k)}$$
with $t_k$ being the k-th time when an event happened.

[therneau]

I've fixed this in my copy, thanks.