Fix sign used for atom-style potential

A positive sign in front of the electric potential is consistent with
E = -grad(electric potential).

src/REAXFF/fix_qeq_reaxff.cpp

@@ -1158,7 +1158,7 @@ void FixQEqReaxFF::get_chi_field()
     for (int i = 0; i < nlocal; i++) {
       if (mask[i] & efgroupbit) {
         if (region && !region->match(x[i][0],x[i][1],x[i][2])) continue;
-        chi_field[i] = -efield->efield[i][3];
+        chi_field[i] = efield->efield[i][3];
       }
     }
   }