Correct thinko in vignette

vignettes/neurons-as-graph.Rmd

@@ -85,7 +85,7 @@ for (i in orders) {
   plot(subset(n, so$points==i), col=i, add = i!=1, boundingbox = boundingbox(n))
 }
 ```
-Note the use of multiple calls to `plot.neuron` using the `add=FALSE` argument 
+Note the use of multiple calls to `plot.neuron` using the `add=TRUE` argument 
 for all but the first plot. Note also the use of the `boundingbox` argument/function 
 in order to ensure that the plot is set up with appropriate axes for the whole
 neuron even if only part of it is plotted in the first call to `plot`.