THM: bugfix T_dot term: 1->alpha_B

According to the derivation of the mass balance eq. in https://www.nature.com/articles/s41467-019-12146-0 (Eq. 22 which contains a sign error and does not contain alpha but would if Eq. 13 is correctly plugged in into Eq. 19) and the equations of Lewis/Schreffler 1998 (The Finite Element Method in the Static and Dynamic Deformation and Consolidation of Porous Media) eg., p. 356, I believe that the Biot coefficient is missing in the solid part of the T_dot term.