Fix link to service desk configuration help doc

What does this MR do and why?

Fixes broken promotion link to service desk configuration help which is behind the Configure Service Desk button in the following message presented at Project Settings, General, Service Desk when Service Desk is not enabled:

This currently links to https://gitlab.example.com/help/user/project/service_desk.html#configuring-service-desk which returns a 404 Page Not Found error.

The new link (following changes made in !142641 (merged)) is https://gitlab.example.com/help/user/project/service_desk/configure

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Before	After

How to set up and validate locally

1. without configuring incoming mail or enabling gitlab_rails['service_desk_email_enabled'] in gitlab.rb browse to Project Settings, General, Service Desk

2. You should be presented with a promotional message (if nothing is displayed remove the promote_service_desk_dismissed cookie from your browser session):

   

3. Click the Configure Service Desk button - you should be taken to https://gitlab.example.com/help/user/project/service_desk/configure