Use HTTPS in url (hadley#1129)

index.Rmd

@@ -3,7 +3,7 @@ knit: "bookdown::render_book"
 title: "Advanced R"
 author: ["Hadley Wickham"]
 description: "The book is designed primarily for R users who want to improve their programming skills and understanding of the language.  It should also be useful for programmers coming to R from other languages, as it explains some of R's quirks and shows how some parts that seem horrible do have a positive side."
-url: 'http\://adv-r.hadley.nz/'
+url: 'https\://adv-r.hadley.nz/'
 github-repo: hadley/adv-r
 twitter-handle: hadley
 cover-image: cover.png