Fix an error in abinitio.py's documentation

src/abinitio.py

@@ -606,9 +606,9 @@ def get_energy_force(self, mvals, AGrad=False, AHess=False):
 
  In equation form, the objective function is given by:
 
- \[ = {W_E}\left[ {\frac{{\left( {\sum\limits_{i \in {N_s}}
- {{w_i}{{\left( {E_i^{MM} - E_i^{QM}} \right)}^2}} } \right) -
- {{\left( {{{\bar E}^{MM}} - {{\bar E}^{QM}}} \right)}^2}}}
+ \[ = {W_E}\left[ {\frac{{{\sum\limits_{i \in {N_s}}
+ {{w_i}{{\left( {E_i^{MM} - E_i^{QM}}
+ - \left( {{{\bar E}^{MM}} - {{\bar E}^{QM}}} \right) \right)}^2}}}}}
  {{\sum\limits_{i \in {N_s}} {{w_i}{{\left(
  {E_i^{QM} - {{\bar E}^{QM}}} \right)}^2}} }}} \right] +
  {W_F}\left[ {\frac{{\sum\limits_{i \in {N_s}} {{w_i}\sum\limits_{a \in {N_a}}