Update alerts_reports.mdx (apache#15904)

Slack requires `chat:write` as part of "OAuth & Permissions" for the slack integration to be able to post messages.

docs/src/pages/docs/installation/alerts_reports.mdx

@@ -38,6 +38,7 @@ To send alerts and reports to Slack channels, you need to create a new Slack App
 3. Go to "OAuth & Permissions" section, and give the following scopes to your app:
     - `incoming-webhook`
     - `files:write`
+    - `chat:write`
 4. At the top of the "OAuth and Permissions" section, click "install to workspace".
 5. Select a default channel for your app and continue.
 (You can post to any channel by inviting your Superset app into that channel).