Fix sample Prometheus alert config

What does this MR do?

It updates a piece of the documentation with sample YAML for an alert config to make Prometheus send HTTP webhooks to GitLab. The bearer token config must have changed because it now is under authorization.credentials according to the Prometheus documentation.

Does this MR meet the acceptance criteria?

Conformity

• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• This change is backwards compatible across updates, or this does not apply.