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: I221e1add60e2c2e71ee16556834aaaf1b3621211
(cherry picked from commit a19d122dd2ab0b0ba2d54f41e8f0ff6c24113635)
1 file changed