Several missing words added to vignette

Author: tonytonov

vignettes/extending-ggplot2.Rmd

@@ -177,7 +177,7 @@ ggplot(mpg, aes(displ, hwy)) +
   stat_lm(formula = y ~ poly(x, 10), geom = "point", colour = "red", n = 20)
 ```
 
-Note that don't _have_ to explicitly include the new parameters in the arguments for the layer, `...` will get passed to the right place anyway. But you'll need to document them somewhere so the user knows about them. Here's a brief example. Note `@inheritParams ggplot2::stat_identity`: that will automatically inherit documentation for all the parameters also defined for `stat_identity()`.
+Note that we don't _have_ to explicitly include the new parameters in the arguments for the layer, `...` will get passed to the right place anyway. But you'll need to document them somewhere so the user knows about them. Here's a brief example. Note `@inheritParams ggplot2::stat_identity`: that will automatically inherit documentation for all the parameters also defined for `stat_identity()`.
 
 ```{r}
 #' @inheritParams ggplot2::stat_identity
@@ -199,7 +199,7 @@ stat_lm <- function(mapping = NULL, data = NULL, geom = "line",
 
 ### Picking defaults
 
-Sometimes you have calculations that should performed once for the complete dataset, not once for each group. This is useful for picking sensible default values. For example, if we want to do a density estimate, it's reasonable to pick one bandwidth for the whole plot. The following Stat creates a variation of the `stat_density()` that picks one bandwidth for all groups by choosing the mean of the "best" bandwidth for each group (I have no theoretical justification for this, but it doesn't seem unreasonable).
+Sometimes you have calculations that should be performed once for the complete dataset, not once for each group. This is useful for picking sensible default values. For example, if we want to do a density estimate, it's reasonable to pick one bandwidth for the whole plot. The following Stat creates a variation of the `stat_density()` that picks one bandwidth for all groups by choosing the mean of the "best" bandwidth for each group (I have no theoretical justification for this, but it doesn't seem unreasonable).
 
 To do this we override the `setup_params()` method. It's passed the data and a list of params, and returns an updated list.