fix(env): clarify WEBUI_AUTH comment to avoid reading as a setting line

The previous reword still contained a Docker Compose variable reference
(${WEBUI_AUTH:-true}) inside an .env.example comment, which read like
a setting line and could confuse users about what to do with the file.

Replace with a plain instruction: 'Mantener en true.' (just keep it true).
No functional change - comments are not parsed by deploy.sh or compose.

.env.example

@@ -14,7 +14,7 @@ OLLAMA_PORT=11434
 
 # Si false, desactiva el login de Open WebUI.
 # Recomendado solo para uso personal en local.
-# NO expongas el puerto a internet sin autenticacion. Por defecto WEBUI_AUTH=***${WEBUI_AUTH:-true}.
+# NO expongas el puerto a internet sin autenticacion. Mantener en true.
 # Valores validos: true | false
 WEBUI_AUTH=true