Fix missing zone volume in Engineering Reference C_z definition

[brianlball]

Fixes #11418

Add missing zone volume (V_z) term to Engineering Reference Equation 2.1 definition of C_z.

The definition was ρ_air × C_p × C_T (units: J/°C·m³). Correct definition is V_z × ρ_air × C_p × C_T (units: J/°C),
matching the code in ZoneTempPredictorCorrector.cc.

Documentation-only change — no code or simulation impact.

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[brianlball]

existing docs

[rraustad]

This does look correct to me. The right hand side is Watts. So left hand side is Cz = J/C and dTz/dt = C/s, so J/s = W.

[brianlball]

thank you @rraustad !

[brianlball]

after the fix

[mitchute]

Where's your unit test... tsk tsk... 😄 jk. Merging.