Fix slack kibana properties

Made the schema valid according to draft-07

Author: varunvora

elastalert/schema.yaml

@@ -286,9 +286,9 @@ properties:
   slack_text_string: {type: string}
   slack_ignore_ssl_errors: {type: boolean}
   slack_ca_certs: {type: string}
-  slack_attach_kibana_discover_url {type: boolean}
-  slack_kibana_discover_color {type: string}
-  slack_kibana_discover_title {type: string}
+  slack_attach_kibana_discover_url: {type: boolean}
+  slack_kibana_discover_color: {type: string}
+  slack_kibana_discover_title: {type: string}
 
   ### Mattermost
   mattermost_webhook_url: *arrayOfString