fix: add "operator" arg to Slack_channel alert

Issue: #59

docs/resources/logdna_alert.md

@@ -141,6 +141,7 @@ The following arguments are supported by `logdna_alert`:
 `slack_channel` supports the following arguments:
 
 - `immediate`: **_string_** _(Optional; Default: `"false"`)_ Whether the Alert will trigger immediately after the trigger limit is reached. Valid options are `"true"` and `"false"` for presence Alerts and `"false"` for absence Alerts.
+- `operator`: **_string_** _(Optional; Default: `presence`)_ Whether the Alert will trigger on the presence or absence of logs. Valid options are `presence` and `absence`.
 - `terminal`: **_string_** _(Optional; Default: `"true"`)_ Whether the Alert will trigger after the `triggerinterval` if the Alert condition is met (e.g., send an Alert after 30s). Valid options are `"true"` and `"false"` for presence Alerts and `"true"` for absence Alerts.
 - `triggerinterval`: **_string_** _(Optional; Defaults: `"30"` for presence; `"15m"` for absence)_ Interval which the Alert will be looking for presence or absence of log lines. For presence Alerts, valid options are: `30`, `1m`, `5m`, `15m`, `30m`, `1h`, `6h`, `12h`, and `24h`. For absence Alerts, valid options are: `15m`, `30m`, `1h`, `6h`, `12h`, and `24h`.
 - `triggerlimit`: **_integer (Required)_** Number of lines before the Alert is triggered (e.g. setting a value of `10` for an `absence` Alert would alert you if `10` lines were not seen in the `triggerinterval`).