fix plugintest example and docs (#9213)

plugin/plugintest/doc.go

@@ -0,0 +1,11 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+// The plugintest package provides mocks that can be used to test plugins.
+//
+// The mocks are created using testify's mock package:
+// https://godoc.org/github.com/stretchr/testify/mock
+//
+// If you need to import the mock package, you can import it with
+// "github.com/mattermost/mattermost-server/plugin/plugintest/mock".
+package plugintest