Humanoid-Walk task bug report

Thank you for your contribution.
I found a little bug in `mjpc/tasks/humanoid/walk/task.xml` and `walk.cc`
In line 33 and 34 of `task.xml`
```


```
The definition of "velocity" is before the definition of "walk"
However in "walk.cc".
```
// com vel
double* waist_lower_subcomvel =
SensorByName(model, data, "waist_lower_subcomvel");
double* torso_velocity = SensorByName(model, data, "torso_velocity");
double com_vel[2];
mju_add(com_vel, waist_lower_subcomvel, torso_velocity, 2);
mju_scl(com_vel, com_vel, 0.5, 2);

// walk forward
residual[counter++] =
standing * (mju_dot(com_vel, forward, 2) - parameters_[1]);

// ----- move feet ----- //
double* foot_right_vel = SensorByName(model, data, "foot_right_velocity");
double* foot_left_vel = SensorByName(model, data, "foot_left_velocity");
double move_feet[2];
mju_copy(move_feet, com_vel, 2);
mju_addToScl(move_feet, foot_right_vel, -0.5, 2);
mju_addToScl(move_feet, foot_left_vel, -0.5, 2);

mju_copy(&residual[counter], move_feet, 2);
mju_scl(&residual[counter], &residual[counter], standing, 2);
counter += 2;
```
It is reversed.