rs-ar-basic: fix extrinsic pose to camera transformation

[radfordi]

Found by user @JBBee: #4883 (comment).

[dorian3d]

@radfordi Could you also update this comment:

// This is the pose of the fisheye sensor relative to the T265 coordinate system.

It should say the opposite:

// This is the pose of the T265 coordinate system relative to the fisheye sensor

[radfordi]

Thanks @dorian3d. I changed the variable name to replace the comment. That way it makes the usage site clearer as well.

[dorian3d]

Thanks @radfordi. I forgot about the readme. Do you mind updating it too?