doc: Small change to documentation in DectectPython.cmake
doc: Small change to documentation in DectectPython.cmake

Fixed small typo for python 3 default version.

Signed-off-by: Noel Eck <1e36ab6ededef40f3f16bdbc1b09eee72c5d08cf@intel.com>

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv