Update dev site build step (#12190)

Need to set `PROJECT=dev`, instead of `RTD_DOCSET=dev`

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:books: Documentation previews :books:

- User's documentation (`docs`):
https://docs--12190.org.readthedocs.build/12190/

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<!-- readthedocs-preview dev start -->
- Developer's documentation (`dev`):
https://dev--12190.org.readthedocs.build/12190/

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Signed-off-by: Pavithra Eswaramoorthy <[email protected]>

Author: pavithraes

docs/dev/docs.rst

@@ -64,6 +64,6 @@ you may want to verify those changes locally before pushing upstream.
    .. code-block:: console
 
       (.venv) $ cd docs
-      (.venv) $ RTD_DOCSET=dev make livehtml
+      (.venv) $ PROJECT=dev make livehtml
 
 #. Check the changes locally at http://127.0.0.1:4444/ (rebuilds each time you edit and save a file).