SMINN_TO_PLANT_FUN units incorrect in history

### Brief summary of bug

A user suggests that SMINN_TO_PLANT_FUN should have units gN/m3/s in history rather than gN/m2/s.
From a quick look with @wwieder and @olyson, they agree.
The discussion is here:
https://bb.cgd.ucar.edu/cesm/threads/output-variables-from-fun-model.9016/post-52292
Before correcting, I would like a careful look to confirm, unless others are sure beyond doubt.

### General bug information

**CTSM version you are using:**
ctsm5.1.dev153

**Does this bug cause significantly incorrect results in the model's science?**
NO

### Details of bug

`biogeochem/CNVegNitrogenFluxType.F90: call hist_addfld1d (fname='SMINN_TO_PLANT_FUN', units='gN/m^2/s',&`