Docks: Update provisioning.md with proper Slack settings (#21227)

From the below link seems that these are the proper slack settings, and I was able to test/verify. 
https://github.com/grafana/grafana/blob/master/pkg/services/alerting/notifiers/slack.go

docs/sources/administration/provisioning.md

@@ -348,8 +348,8 @@ The following sections detail the supported settings for each alert notification
 | url |
 | recipient |
 | username |
-| iconEmoji |
-| iconUrl |
+| icon_emoji |
+| icon_url |
 | uploadImage |
 | mention |
 | token |