Update Mattermost slash commands configuration labels to match current...

Update Mattermost slash commands configuration labels to match current (5.24.2); fix typo (Override -> Enable).

doc/user/project/integrations/mattermost_slash_commands.md

@@ -48,9 +48,9 @@ the administrator console.
 
    ![Mattermost go to console](img/mattermost_goto_console.png)
 
-1. Click **Custom integrations** and set **Enable Custom Slash Commands**,
-   **Enable custom integrations to override usernames**, and **Override
-   custom integrations to override profile picture icons** to true
+1. Click **Integration Management** and set **Enable Custom Slash Commands**,
+   **Enable integrations to override usernames**, and **Enable
+   integrations to override profile picture icons** to true
 
    ![Mattermost console](img/mattermost_console_integrations.png)