Fix online data migrations cmd in release notes

The online data migrations command in the release notes
needs to be corrected.

$ cinder-manage db online-data-migrations
usage: cinder-manage db [-h]
{online_data_migrations,purge,sync,version} ...
cinder-manage db: error: argument action: invalid choice:
'online-data-migrations' (choose from 'online_data_migrations',
                          'purge', 'sync', 'version')

"cinder-manage db online_data_migrations" is the right command.

Change-Id: I38d36f0721a6d0889639010e91ab041f935d0a00

releasenotes/notes/cinder-manage-db-online-schema-migrations-d1c0d40f26d0f033.yaml

@@ -4,7 +4,7 @@ upgrade:
     Cinder will now be able to execute them online, on a live cloud. Before
     upgrading from Ocata to Pike, operator needs to perform all the Newton
     data migrations. To achieve that he needs to perform `cinder-manage db
-    online-data-migrations` until there are no records to be updated. To limit
+    online_data_migrations` until there are no records to be updated. To limit
     DB performance impact migrations can be performed in chunks limited by
     `--max_number` option. If your intent is to upgrade Cinder in a non-live
     manner, you can use `--ignore_state` option safely. Please note that

releasenotes/notes/migrate-cg-to-generic-volume-groups-f82ad3658f3e567c.yaml

@@ -1,5 +1,5 @@
 ---
 upgrade:
   - Operator needs to perform `cinder-manage db
-    online-data-migrations` to migrate existing consistency
+    online_data_migrations` to migrate existing consistency
     groups to generic volume groups.