As pointed out in https://github.com/mongodb/mongo-python-driver/pull/1186, the docs page incorrectly says to use urllib.parse.quote() when it should say urllib.parse.quote_plus():

Username and password must be percent-escaped with urllib.parse.quote(), to be used in a MongoDB URI.

https://pymongo.readthedocs.io/en/stable/examples/authentication.html?highlight=quote#percent-escaping-username-and-password

This was missed while fixing PYTHON-2973.