Update visualize.Rmd (#182)

changed "This defines how variables in your dataset and mapped to..." to "This defines how variables in your dataset are mapped to..."

visualize.Rmd

@@ -67,7 +67,7 @@ With ggplot2, you begin a plot with the function `ggplot()`. `ggplot()` creates
 
 You complete your graph by adding one or more layers to `ggplot()`. The function `geom_point()` adds a layer of points to your plot, which creates a scatterplot. ggplot2 comes with many geom functions that each add a different type of layer to a plot. You'll learn a whole bunch of them through out this chapter.
 
-Each geom function in ggplot2 takes a `mapping` argument. This defines how variables in your dataset and mapped to visual properties. You must always use `mapping()` in conjunction with `aes()`. The `x` and `y` arguments of `aes()` describe which variables to map to the x and y axes of your plot, and ggplot2 will look for those variables in your dataset, `mpg`.
+Each geom function in ggplot2 takes a `mapping` argument. This defines how variables in your dataset are mapped to visual properties. You must always use `mapping()` in conjunction with `aes()`. The `x` and `y` arguments of `aes()` describe which variables to map to the x and y axes of your plot, and ggplot2 will look for those variables in your dataset, `mpg`.
 
 Let's turn this code into a reusable template for making graphs with ggplot2. To make a graph, replace the bracketed sections in the code below with a dataset, a geom function, or a set of mappings.