Fix docs build after #33241

Recently-merged PR #33241 broke the docs build, and this fixes it.

Author: DaveCTurner

docs/reference/modules/discovery/zen.asciidoc

@@ -68,14 +68,14 @@ startup.
 
 To enable file-based discovery, configure the `file` hosts provider as follows:
 
-```
+[source,txt]
+----------------------------------------------------------------
 discovery.zen.hosts_provider: file
-```
+----------------------------------------------------------------
 
-Then create a file at `$ES_PATH_CONF/unicast_hosts.txt` in
-<<discovery-file-format,the format described below>>. Any time a change is made
-to the `unicast_hosts.txt` file the new changes will be picked up by
-Elasticsearch and the new hosts list will be used.
+Then create a file at `$ES_PATH_CONF/unicast_hosts.txt` in the format described
+below. Any time a change is made to the `unicast_hosts.txt` file the new
+changes will be picked up by Elasticsearch and the new hosts list will be used.
 
 Note that the file-based discovery plugin augments the unicast hosts list in
 `elasticsearch.yml`: if there are valid unicast host entries in
@@ -86,10 +86,6 @@ The `discovery.zen.ping.unicast.resolve_timeout` setting also applies to DNS
 lookups for nodes specified by address via file-based discovery. This is
 specified as a <<time-units, time unit>> and defaults to 5s.
 
-[[discovery-file-format]]
-[float]
-====== unicast_hosts.txt file format
-
 The format of the file is to specify one node entry per line.  Each node entry
 consists of the host (host name or IP address) and an optional transport port
 number.  If the port number is specified, is must come immediately after the