Fix main.yml

Use rstudio's repos.

inst/templates/main.yml

@@ -22,7 +22,7 @@ jobs:
             && sh get-docker.sh
       - name: Install package dependencies
         run: |
-          Rscript -e "install.packages(c('pkgdown', 'remotes', 'rcmdcheck'), repos = 'https://demo.rstudiopm.com/all/__linux__/bionic/latest')"
+          Rscript -e "install.packages(c('pkgdown', 'remotes', 'rcmdcheck'), repos = 'https://cran.rstudio.com/')"
           Rscript -e "remotes::install_deps(dependencies = TRUE)"
       - name: Check package
         run: |