doc: Change variable name DEFAULT_API_VERSION to _MIN_API_VERSION
The DEFAULT_API_VERSION variable doesn't exist in the specified
file.
As stated in [1], _MIN_API_VERSION is used as the default api
version if no version is specified.
[1]
@@ -20,7 +20,7 @@ word ``volume``::
OpenStack-API-Version: volume 3.0
If a user makes a request without specifying a version, they will get
-the ``DEFAULT_API_VERSION`` as defined in
+the ``_MIN_API_VERSION`` as defined in
``cinder/api/openstack/api_version_request.py``. This value is currently ``3.0``
and is expected to remain so for quite a long time.
