fix: using correct focal length variable for last example

[alexflorensa]

In the parameter declaration of the last example, FOCAL_LENGTH is used, but in the following part of the code, F is used as focal length:

Z = PrincipalAxis(
    camera_center=C,
    camera_dz=camera_frame.dz,
    f=F,
)

This PR fixes the above code to make use of the FOCAL_LENGTH variable, instead of F, that refers to a previous example.

[mnslarcher]

Thanks @alexflorensa !