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: I85af8d4c95e721de9b32d5bb180b46ec712db06b
1 file changed