Update r4ds url

- This pull request updates r4ds(<https://r4ds.had.co.nz/>) to r4ds 2e (https://r4ds.hadley.nz/).

README.Rmd

@@ -24,7 +24,7 @@ library(stringr)
 
 ## Overview
 
-Strings are not glamorous, high-profile components of R, but they do play a big role in many data cleaning and preparation tasks. The stringr package provides a cohesive set of functions designed to make working with strings as easy as possible. If you're not familiar with strings, the best place to start is the [chapter on strings](https://r4ds.had.co.nz/strings.html) in R for Data Science.
+Strings are not glamorous, high-profile components of R, but they do play a big role in many data cleaning and preparation tasks. The stringr package provides a cohesive set of functions designed to make working with strings as easy as possible. If you're not familiar with strings, the best place to start is the [chapter on strings](https://r4ds.hadley.nz/strings) in R for Data Science.
 
 stringr is built on top of [stringi](https://github.com/gagolews/stringi), which uses the [ICU](https://icu.unicode.org) C library to provide fast, correct implementations of common string manipulations. stringr focusses on the most important and commonly used string manipulation functions whereas stringi provides a comprehensive set covering almost anything you can imagine. If you find that stringr is missing a function that you need, try looking in stringi. Both packages share similar conventions, so once you've mastered stringr, you should find stringi similarly easy to use.