kinematics_correlations.py: ak.count -> ak.num where needed

Fixes failure seen in https://eicweb.phy.anl.gov/EIC/benchmarks/physics_benchmarks/-/jobs/1755651

>>> import awkward as ak
>>> ak.count([[1], [2]], axis=0)
<Array [2] type='1 * int64'>
>>> ak.num([[1], [2]], axis=0)
2