Update search-across-clusters.asciidoc to reflect the true default value of skip_unavailable setting. (elastic#120592) (elastic#120631)

Author: astefan

docs/reference/search/search-your-data/search-across-clusters.asciidoc

@@ -109,7 +109,7 @@ PUT _cluster/settings
 // end::ccs-remote-cluster-setup[]
 
 <1> Since `skip_unavailable` was not set on `cluster_three`, it uses
-the default of `false`. See the <<skip-unavailable-clusters>>
+the default of `true`. See the <<skip-unavailable-clusters>>
 section for details.