Update messages/agent.preview.md

Co-authored-by: Juliet Shackell <63259011+jshackell-sfdc@users.noreply.github.com>

Author: WillieRuemmele

messages/agent.preview.md

@@ -49,7 +49,7 @@ Session ID of an in-progress preview session; use with --utterance to send a mes
 
 # error.sessionIdRequiresUtterance
 
---session-id must be used with --utterance.
+When using --session-id, you must also specify --utterance.
 
 # error.utteranceRequiresAgent