When I tried to load the URDF file on MuJoCo, it failed with the
following error message:
Duplicated slider name "slider_left"
It turned out that the issue was the right slider being wrongly
tagged as 'slider_left' (not 'slider_right'). Fix it thusly.
Signed-off-by: Fujimoto Seiji <fujimoto@ceptord.net>