Update section 4. of Setup Guide to use BOT Account

[DHaussermann]

The setup guide should to be updated to use the new BOT accounts functionality. This functionality will be exposed in the UI for 5.12

Section 4. - step ii.
Create a new Mattermost user, through the regular UI or the CLI with the username "gitlab"

This should be updated for 5.12 to instruct the user to navigate to the integrations section and create a new bot instead of a user account that functions as a BOT.