[doc] remove reference to seal, was removed in #11336

docs/reference/indices.asciidoc

@@ -107,8 +107,6 @@ include::indices/refresh.asciidoc[]
 
 include::indices/optimize.asciidoc[]
 
-include::indices/seal.asciidoc[]
-
 include::indices/shadow-replicas.asciidoc[]
 
 include::indices/upgrade.asciidoc[]