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: Icdfca5f41e8520c06413bf30c76ff0ad2d338980
(cherry picked from commit f307043cb7acd8d6fd68c815bcd71df535b06064)
1 file changed