Correct minor typos in doc (chaos-polymtl#1245)

Description
Minor corrections in the MG documentation.

Former-commit-id: 8aa6dc9

Author: lpsaavedra

include/solvers/fluid_dynamics_matrix_free.h

@@ -126,15 +126,15 @@ class MFNavierStokesPreconditionGMG
 
 private:
   /**
-   * @brief Set the up AMG object needed for coarse-grid solver or
+   * @brief Set up AMG object needed for coarse-grid solver or
    * preconditioning.
    *
    */
   void
   setup_AMG();
 
   /**
-   * @brief Set the up ILU object needed for coarse-grid solver or
+   * @brief Set up ILU object needed for coarse-grid solver or
    * preconditioning.
    *
    */