Update docs/source/developer/extension_dev.rst
Update docs/source/developer/extension_dev.rst

Co-Authored-By: Paul Ivanov <b02e5b66ace6dc3b459be661062c452b50ea1c13@berkeley.edu>
jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab