doc: upgrade Sphinx version to resolve doc build error

Doc build fails on ubuntu 22.04 due to Sphinx 4.0.2 compatibility issue
with default Python 3.10. Upgrade Sphinx version to resolve the issue.

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Change-Id: Ia80dc648bc02bd0887ff1a739b88a14759e7fec6