[Website]: FortiGate setup snippet recommends invalid CLI value `set format rfc6587`

### Before you submit

- [x] This issue is about a documentation page, flow, or piece of content.

### Type of issue

Inaccurate

### What documentation page or section is affected

https://www.elastic.co/docs/reference/integrations/fortinet_fortigate#vendor-set-up-steps

### What happened?

The snippet under **Vendor set up steps > Syslog Configuration > CLI Configuration** uses a value FortiOS does not accept:

set format rfc6587

`rfc6587` is not a valid value for `set format`. The FortiOS admin guide lists `{default | csv | cef | rfc5424 | json}` for 7.x plus `custom` in 8.0. The CLI rejects it.

RFC 6587 is TCP framing, not a message format, and it is not configurable on the FortiGate: it already applies automatically with `set mode reliable`, which the FortiOS CLI reference defines as "enable reliable syslogging by RFC6587".

The correct value is `set format default`, which is what the pipeline parses. This is worth fixing because users hitting the CLI rejection tend to pick `rfc5424`. With `rfc5424` the FortiGate stops sending `devid`, and since the pipeline renames it to `observer.serial_number` with `ignore_missing: true`, the serial number silently disappears with no error and no tag.

Suggested fix:

set format default

The **Troubleshooting > TCP framing issues** bullet has the same problem: it says both the FortiGate and the integration must have framing set to `rfc6587`, but there is nothing to set on the FortiGate.

### Additional info

_No response_

Open the FortiGate integration page at the linked URL and inspect Vendor set up steps > Syslog Configuration > CLI Configuration, then check Troubleshooting > TCP framing issues for the matching guidance. Confirm both sections reflect the FortiOS CLI reference and that the pipeline’s expected format and framing behavior are described consistently.

Written by the indexing model from the issue text.