Fix bug in primordial chemistry chemical heating

[psharda]

This bug fix needs to be propagated across the primordial chemistry pipeline: https://github.com/psharda/GPUAstroChem/commit/ea95fb62d3b79fa18db5e5dc796c5974d8767bea

It does not matter for primordial chemistry, but creates a 10% error at low, non-zero metallicities. Just noting it down for now in case I forget to fix it later on.