guides: update dex (#24603)

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## Description

Aligned file name with that on line 43, using the preferred
`compose.yaml`.

## Related issues or tickets

Closes #23860

## Reviews

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- [ ] Editorial review

Signed-off-by: Craig Osterhout <craig.osterhout@docker.com>

Author: craig-osterhout

content/guides/dex.md

@@ -87,7 +87,7 @@ or use CLI tools like [htpasswd](https://httpd.apache.org/docs/2.4/programs/htpa
 
 With Docker Compose configured, start Dex:
 ```yaml
-# docker-compose.yaml
+# compose.yaml
 
 services:
   dex: