Broken link to contributers.md from online documentation

[jfunction]

CONTRIBUTING.md was moved into the ./.github directory in https://github.com/rstudio/shiny/commit/ececdf42a74309750ed82f7f9512ea1465ef7bbc

But the contributing guidelines link at https://shiny.rstudio.com/contribute/ links to the old location at https://github.com/rstudio/shiny/blob/master/CONTRIBUTING.md I'm not sure where else to post this issue but expect it is a quick fix, just add /.github to the link on that page.