update pkgdown workflow

Author: ecmerkle

.github/workflows/pkgdown.yaml

@@ -27,7 +27,9 @@ jobs:
       - uses: r-lib/actions/setup-r@v2
         with:
           use-public-rspm: true
-          extra-repositories: "https://mc-stan.org/r-packages/, https://ecmerkle.github.io/drat/"
+          extra-repositories: |
+            https://mc-stan.org/r-packages/ |
+            https://ecmerkle.github.io/drat/
 
       - uses: r-lib/actions/setup-r-dependencies@v2
         with: