DOC: SingelThreaded -> SingleThreaded in Doxygen Group.

Change-Id: Ideccede3bc90f5a5609a22281aab95eefe6aabbf
Suggested-by: Julien Finet <julien.finet@kitware.com>
Merge topic 'SingelDoc'

03b1e81 DOC: SingelThreaded -> SingleThreaded in Doxygen Group.