docs: remove extra period at end of copyright

Sphinx inserts a period at end of Copyright section. This causes the
copyright section to be published as "Arm Limited..". Removing this
period fixes this to "Arm Limited."

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>
Change-Id: I22d229f383668e51483a59dfdd4e0b342abf3bab