Fix system event invocationFailed field name

docs/system-events-and-plugin-system.md

@@ -10,13 +10,16 @@ of event processing flow starting from receiving event to function invocation en
   * `path` - Events API path
   * `headers` - HTTP request headers
 * `eventgateway.function.invoking` - the event emitted before invoking a function. Data fields:
+  * `space` - space name
   * `event` - event payload
   * `functionId` - registered function ID
 * `eventgateway.function.invoked` - the event emitted after successful function invocation. Data fields:
+  * `space` - space name
   * `event` - event payload
   * `functionId` - registered function ID
   * `result` - function response
 * `eventgateway.function.invocationFailed` - the event emitted after failed function invocation. Data fields:
+  * `space` - space name
   * `event` - event payload
   * `functionId` - registered function ID
   * `error` - invocation error

event/system.go

@@ -43,5 +43,5 @@ type SystemFunctionInvocationFailedData struct {
 	Space      string      `json:"space"`
 	FunctionID function.ID `json:"functionId"`
 	Event      Event       `json:"event"`
-	Error      error       `json:"result"`
+	Error      error       `json:"error"`
 }