Add bounds to sphinx requirement

As ONAP was created before Oct 20 2020 in RTD, we
must set bounds to enforce a compatible Sphinx version
(see manual).

Issue-ID: DOC-782

Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
Change-Id: I439dd59c09e0d169b7da8f5153d258d4c9b75200
1 file changed