Change variable name horizFovDegrees in CameraPersp::setPerspective() into verticalFovDegrees

In function `CameraPersp::setPerspective (float horizFovDegrees, float aspectRatio, float nearPlane, float farPlane)`, the `horizFovDegrees` appears to be vertical Fov degrees.

Discussion at the forum,
https://forum.libcinder.org/#Topic/23286000001866015