Revert "Bump actions/upload-pages-artifact from 2 to 3"

This reverts commit 87f7e02.

Author: sim642

.github/workflows/docs.yml

@@ -55,7 +55,7 @@ jobs:
         run: opam exec -- dune build @doc
 
       - name: Upload artifact
-        uses: actions/upload-pages-artifact@v3
+        uses: actions/upload-pages-artifact@v2
         with:
           path: _build/default/_doc/_html/