Corrected field description in Gadget-2 particle definition.

src/hydro/Gadget2/hydro_part.h

@@ -109,7 +109,7 @@ struct part {
       /*! "Grad h" term */
       float f;
 
-      /* Pressure over density squared (including drho/dh term) */
+      /* Pressure over density squared  */
       float P_over_rho2;
 
       /* Particle sound speed. */