fix nordr example

[swihart]

Current line 126 of nordr.R

#' \dontrun{
#' # fit continuation ratio model with nordr and as a logistic model
#' nordr(y, mu=~length, weights=fr, pmu=c(0,-1.4,-2.3), pint=0.13,
#' 	dist="cont")
#' 	}

The problem is that length is coming in and not being recognized as a factor. That causes the error about the pmu argument being too long.