Fix 404 in CONTRIBUTING.md (#789)

Author: etiennebacher

.github/CONTRIBUTING.md

@@ -16,7 +16,7 @@ bug, you may also create an associated issue. If possible, try to illustrate you
 *  Please create a Git branch for each pull request (PR).
 *  Your contributed code should roughly follow the [R style guide](http://style.tidyverse.org), but in particular our [**easystats convention of code-style**](https://easystats.github.io/easystats/articles/conventions.html).
 *  performance uses [roxygen2](https://cran.r-project.org/package=roxygen2), with
-[Markdown syntax](https://cran.r-project.org/web/packages/roxygen2/vignettes/markdown.html),
+[Markdown syntax](https://cran.r-project.org/web/packages/roxygen2/vignettes/rd-formatting.html),
 for documentation.
 *  performance uses [testthat](https://cran.r-project.org/package=testthat). Adding tests to the PR makes it easier for me to merge your PR into the code base.
 *  If your PR is a user-visible change, you may add a bullet to the top of `NEWS.md` describing the changes made. You may optionally add your GitHub username, and links to relevant issue(s)/PR(s).