Fix the API Microversions's doc
Fix the API Microversions's doc

HTTPNotAcceptableAPIVersion has be defined in http_error,
and the error code is 406.

Change-Id: Ieedbef808daf56f7900bba6ce7e797457ca29307

openstack/magnum,openstack/magnum,ArchiFleKs/magnum,ArchiFleKs/magnum