[Rd] nonlinear fitting documentation error (PR#9749)

[Ben Bolker]

 <bkerin <at> fastmail.fm> writes:

> The problem is that fn is not the model to be fitted.  
> The model is y = p1*x/(p2
> + x).  
  Actually, I would say the model is

y_i ~ Normal(p1*x_i/(p2+x_i),sigma^2) ...