ref doc format fix (#2555)

missing a +

solr/solr-ref-guide/modules/deployment-guide/pages/collection-management.adoc

@@ -1648,6 +1648,7 @@ This parameter has no effect if `incremental=false` is specified.
 Indicates if configset files should be included with the index backup or not. Note that in order to restore a collection, the configset must either exist in ZooKeeper or be part of the backup. Only set this to `false` if you can restore configsets by other means external to Solr (i.e. you have it stored with your application source code, is part of your ZooKeeper backups, etc).
 
 `property.<propertyName>` (V1), `extraProperties` (V2)::
++
 [%autowidth,frame=none]
 |===
 |Optional |Default: none