Fix SlackWebhookOperator docs (apache#3915)

The docs refer to `conn_id` while the actual argument is `http_conn_id`.

airflow/contrib/operators/slack_webhook_operator.py

@@ -31,8 +31,8 @@ class SlackWebhookOperator(SimpleHttpOperator):
     Each Slack webhook token can be pre-configured to use a specific channel, username and
     icon. You can override these defaults in this hook.
 
-    :param conn_id: connection that has Slack webhook token in the extra field
-    :type conn_id: str
+    :param http_conn_id: connection that has Slack webhook token in the extra field
+    :type http_conn_id: str
     :param webhook_token: Slack webhook token
     :type webhook_token: str
     :param message: The message you want to send on Slack