Correction to twist in pose_gt plugin to match Odometry msg specifications

[davidorchansky]

The pose_gt plugin publishes the twist values in world frame of reference. However, the specification of the Odometry message state that:

The twist in this message should be specified in the coordinate frame given by the child_frame_id

There seems to be a similar fix done to the p3d sensor plugin in the gazebo package, although it hasn't been merged yet: https://github.com/ros-simulation/gazebo_ros_pkgs/pull/1283