tutorial: skip passing .db for backup option for ovn_start_ovsdb_server:

tutorial: skip passing .db for backup option for ovn_start_ovsdb_server:

current params uses two sb1.db which is redundant:
e.g. ovsdb-server --remote=punix:sb1.ovsdb sb1.db sb1.db
expected:
e.g. ovsdb-server --remote=punix:sb1.ovsdb sb1.db

tested and works as expected:
ovn-sbctl --db=unix:/root/ovs/tutorial/sandbox/sb2.ovsdb show
Chassis "chassis-1"
    hostname: sandbox
    Encap geneve
        ip: "127.0.0.1"
        options: {csum="true"}

Signed-off-by: aginwala <aginwala@ebay.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>