ovn-nb.xml: Fix the description for LB's skip_snat option.

lb_force_snat_ip is a flag set in logical flow pipeline, while
lb_force_snat_ip is the option configured in NB DB.  In NB document we
should mention the actual option configured in NB instead of the flow
details.

Acked-by: Numan siddique <numans@ovn.org>
Signed-off-by: Han Zhou <hzhou@ovn.org>

ovn-nb.xml

@@ -1712,8 +1712,9 @@
 
       <column name="options" key="skip_snat">
         If the load balancing rule is configured with <code>skip_snat</code>
-        option, the force_snat_for_lb option configured for the router
-        pipeline will not be applied for this load balancer.
+        option, the option lb_force_snat_ip configured for the logical router
+        that references this load balancer will not be applied for this load
+        balancer.
       </column>
 
       <column name="options" key="add_route">