Fix path to upload artifact

Author: finsberg

.github/workflows/build_docs.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-22.04
     container: ghcr.io/fenics/dolfinx/lab:v0.9.0
     env:
-      PUBLISH_DIR: ./_build/html
+      PUBLISH_DIR: ./build
       PYVISTA_OFF_SCREEN: false
       PYVISTA_JUPYTER_BACKEND: "html"