Wrong parameter type in centroidal MPC documentation

[CarlottaSartore]

The documentation for the centroidal mpc parameter has some issues (at least if one want to set the parameter via code rather than via conf file):

bipedal-locomotion-framework/src/ReducedModelControllers/include/BipedalLocomotion/ReducedModelControllers/CentroidalMPC.h

Lines 70 to 71 in 9a25588

	* | `sampling_time` | `double` | Sampling time of the MPC. | Yes |
	* | `time_horizon` | `double` | The time horizon of the MPC. The number of knots will be given by `floor(time_horizon / sampling_time)` | Yes |

the time_horizon and sampling_time variable are datetime rather than double.

In addition, the unit is missing (I assume they are seconds).

@GiulioRomualdi if you want (and agree) I can fix them either in #650 or open a new PR ! Let me know

[GiulioRomualdi]

You can open a PR! Thank you 😀