PLT-5873 Renamed private groups to private channels (#5924)

* PLT-5873 Renamed private groups to private channels in web app

* PLT-5873 Renamed private groups to private channels in server

webapp/components/tutorial/tutorial_intro_screens.jsx

@@ -162,7 +162,7 @@ export default class TutorialIntroScreens extends React.Component {
                 <FormattedHTMLMessage
                     id='tutorial_intro.screenTwo'
                     defaultMessage='<h3>How Mattermost works:</h3>
-                    <p>Communication happens in public discussion channels, private groups and direct messages.</p>
+                    <p>Communication happens in public discussion channels, private channels and direct messages.</p>
                     <p>Everything is archived and searchable from any web-enabled desktop, laptop or phone.</p>'
                 />
                 {appDownloadLink}