Fixing bug in NAryMatIterator documentation(Bug #3787).
Fixing bug in NAryMatIterator documentation(Bug #3787).

Fixed the non-working code example for NAryMatIterator.
The documentation of NAryMatIterator in file:
modules/core/include/opencv2/core/core.hpp has a working example which
was used for these edits.

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