Update intro.Rmd

Author: olivroy

vignettes/articles/intro.Rmd

@@ -10,4 +10,4 @@ knitr::opts_chunk$set(collapse = TRUE)
 library(s2)
 ```
 
-See vignette 7, "Spherical geometry in sf using s2geometry" of package sf.
+See [vignette 7, "Spherical geometry in sf using s2geometry"](https://r-spatial.github.io/sf/articles/sf7.html) of package sf.