Corrected typo in isothermal EoS

src/equation_of_state.h

@@ -275,7 +275,7 @@ gas_pressure_from_internal_energy(float density, float u) {
  */
 __attribute__((always_inline)) INLINE static float
 gas_internal_energy_from_pressure(float density, float pressure) {
-  return const_isothermal_energy;
+  return const_isothermal_internal_energy;
 }
 
 /**