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: I5ed268d592986764bf040690a47c5024692768b7
(cherry picked from commit 667217c9a06ec40d2d71ab8f34515f4325631f48)
1 file changed