fixes 2 typos (#4779)

Author: flrd

vignettes/extending-ggplot2.Rmd

@@ -492,7 +492,7 @@ This doesn't allow you to use different geoms with the stat, but that seems appr
 1. Compare and contrast `GeomPolygon` with `GeomSimplePolygon`.
 
 ## Geoms and Stats with multiple orientation
-Some layers have a specific orientation. `geom_bar()` e.g. have the bars along one axis, `geom_line()` will sort the input by one axis, etc. The original approach to using these geoms in the other orientation was to add `coord_flip()` to the plot to switch the position of the x and y axes. Following ggplot2 v3.3 all the geoms will natively work in both orientations without `coord_flip()`. The mechanism is that the layer will try to guess the orientation from the mapped data, or take direction from the user using the `orientation` parameter. To replicate this functionality in new stats and geoms there's a few steps to take. We wll look at the boxplot layer as an example instead of creating a new from scratch.
+Some layers have a specific orientation. `geom_bar()` e.g. have the bars along one axis, `geom_line()` will sort the input by one axis, etc. The original approach to using these geoms in the other orientation was to add `coord_flip()` to the plot to switch the position of the x and y axes. Following ggplot2 v3.3 all the geoms will natively work in both orientations without `coord_flip()`. The mechanism is that the layer will try to guess the orientation from the mapped data, or take direction from the user using the `orientation` parameter. To replicate this functionality in new stats and geoms there's a few steps to take. We will look at the boxplot layer as an example instead of creating a new one from scratch.
 
 ### Omnidirectional stats
 The actual guessing of orientation will happen in `setup_params()` using the `has_flipped_aes()` helper:
@@ -509,7 +509,7 @@ StatBoxplot$setup_data
 
 The data is flipped (if needed), manipulated, and flipped back as it is returned. 
 
-During the computation, this sandwiching between `flip_data()` is used as well, but right before the data is returned it will also get a `flipped_aes` column denoting if the data is flipped or not. This allow the
+During the computation, this sandwiching between `flip_data()` is used as well, but right before the data is returned it will also get a `flipped_aes` column denoting if the data is flipped or not. This allows the
 stat to communicate to the geom that orientation has already been determined.
 
 ### Omnidirecitonal geoms