update tidyverse loadings (#754)

Update to the tidyverse prerequsities for the strings and factors chapters. stringr and forcats now load with the core tidyverse.

strings.Rmd

@@ -6,11 +6,10 @@ This chapter introduces you to string manipulation in R. You'll learn the basics
 
 ### Prerequisites
 
-This chapter will focus on the __stringr__ package for string manipulation. stringr is not part of the core tidyverse because you don't always have textual data, so we need to load it explicitly.
+This chapter will focus on the __stringr__ package for string manipulation, which is part of the core tidyverse.
 
 ```{r setup, message = FALSE}
 library(tidyverse)
-library(stringr)
 ```
 
 ## String basics