Update agent-command-reference.md (elastic#1814)

Remove unused \<hr> tag that was not rendered on the website.

Tyring to fix the screenshot:


![image](https://github.com/user-attachments/assets/ee86bff9-e4be-4fab-ad72-a9e35dd00c60)

Author: lhirlimann

reference/fleet/agent-command-reference.md

@@ -39,7 +39,7 @@ Note the following restrictions for running {{agent}} commands:
 * [unprivileged](#elastic-agent-unprivileged-command)
 * [version](#elastic-agent-version-command)
 
-<hr>
+
 
 ## elastic-agent diagnostics [elastic-agent-diagnostics-command]