bug in sdf_exporter.py

[Pan-Chenliang]

examples/scripts/blender/sdf_exporter.py line 188
sdf_tag = ET.SubElement(model, "sdf", attrib={"sdf": "1.8"})
makes a mistake in gazebo sim, maybe it should be
sdf_tag = ET.SubElement(model, "sdf", attrib={"version": "1.8"})

[azeey]

Thanks! Looks like you have a fix there. Mind opening a PR?