Bump actions/download-artifact from 2 to 3 (#552)

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.github/workflows/build-deploy-docs.yml

@@ -31,7 +31,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Download book
-        uses: actions/download-artifact@v2
+        uses: actions/download-artifact@v3
         with:
           name: book
           path: artifacts/book