Fix URL repo docs for searchable snapshots (elastic#77624)

Today we document that you can use URL repositories with searchable
snapshots, but in fact it only works for HTTP(S) repositories. This
commit adjusts the docs to clarify.

Relates elastic#69521

Author: DaveCTurner

docs/reference/searchable-snapshots/index.asciidoc

@@ -80,7 +80,7 @@ Use any of the following repository types with searchable snapshots:
 * {plugins}/repository-azure.html[Azure Blob Storage]
 * {plugins}/repository-hdfs.html[Hadoop Distributed File Store (HDFS)]
 * <<snapshots-filesystem-repository,Shared filesystems>> such as NFS
-* <<snapshots-read-only-repository,URL repositories>>
+* <<snapshots-read-only-repository,Read-only HTTP and HTTPS repositories>>
 
 You can also use alternative implementations of these repository types, for
 instance