Actually force source install of stringfish

Author: VisruthSK

.github/workflows/pkgdown.yaml

@@ -34,7 +34,7 @@ jobs:
 
       - uses: r-lib/actions/setup-r-dependencies@v2
         with:
-          extra-packages: any::pkgdown, local::., stan-dev/pkgdown-config, any::stringfish?source
+          extra-packages: any::pkgdown, local::., stan-dev/pkgdown-config, stringfish?source
 
       - name: Install CmdStan
         run: Rscript -e "cmdstanr::install_cmdstan()"