Fixed #define typo

DDDS_THREADS_SINGLE should be DDS_THREADS_SINGLE in Scheduler.cpp (three places).
The effect of the bug was very modest.
Revert "Fixed #define typo"

This reverts commit c4fcac12de44966a19b7459f4de18f3009f8f0f4.
Fixed #define typo

DDDS_THREADS_SINGLE should have been DDS_THREADS_SINGLE in three places.
I initially committed this to develop, realized the error of my ways, reversed it, made a
branch "define-typo", and committed to this instead.
Merge pull request #7 from sorenhein/define-typo

Fixed #define typo in Scheduler.cpp