BASE_DIR is only mentioned in the tutorial from the in development
docs, which may not be the best place to start - chiefly because it
can have issues like this.
If you don't mind encountering things like this, then using the dev
docs helps find issues like this that have been added since the last
release, but if you want a more checked and correct django docs, use
the ones from the latest release.
Of course, you should always use the docs for the version that you are
actually using. I would recommend not using unreleased versions of
django to learn django though.
Cheers
Tom