Update examples/custom_struct/packed_matrix.py
Update examples/custom_struct/packed_matrix.py

Update the C++ standard required for the packed matrix example.

Co-authored-by: Leo Fang <3db24426da64ec5cf042481a8ffb3ee6a3dac8c3@bnl.gov>
cupy/cupy,cupy/cupy,cupy/cupy,cupy/cupy