[DOCS] Mark `filter` param as optional

docs/reference/indices/add-alias.asciidoc

@@ -64,7 +64,7 @@ include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=timeoutparms]
 ==== {api-request-body-title}
 
 `filter`::
-(Required, query object)
+(Optional, query object)
 include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=index-alias-filter]
 
 include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=index-routing]