fix minor plaintext typos and links (#1082)

intro.qmd

@@ -153,7 +153,7 @@ You'll need at least R 4.1.0 for this book.
 ### RStudio
 
 RStudio is an integrated development environment, or IDE, for R programming.
-Download and install it from <http://www.rstudio.com/download>.
+Download and install it from <https://www.rstudio.com/download>.
 RStudio is updated a couple of times a year.
 When a new version is available, RStudio will let you know.
 It's a good idea to upgrade regularly so you can take advantage of the latest and greatest features.
@@ -262,7 +262,7 @@ There are a few people we'd like to thank in particular, because they have spent
 
 -   Jenny Bryan and Lionel Henry for many helpful discussions around working with lists and list-columns.
 
--   The three chapters on workflow were adapted (with permission), from <http://stat545.com/block002_hello-r-workspace-wd-project.html> by Jenny Bryan.
+-   The three chapters on workflow were adapted (with permission), from <https://stat545.com/block002_hello-r-workspace-wd-project.html> by Jenny Bryan.
 
 -   Yihui Xie for his work on the [bookdown](https://github.com/rstudio/bookdown) package, and for tirelessly responding to my feature requests.
 
@@ -295,7 +295,7 @@ cat(".\n")
 
 ## Colophon
 
-An online version of this book is available at [http://r4ds.had.co.nz](http://r4ds.hadley.nz){.uri}.
+An online version of this book is available at [https://r4ds.had.co.nz](https://r4ds.hadley.nz){.uri}.
 It will continue to evolve in between reprints of the physical book.
 The source of the book is available at <https://github.com/hadley/r4ds>.
 The book is powered by <https://bookdown.org> which makes it easy to turn R Markdown files into HTML, PDF, and EPUB.