Doc: Name of project bot user can have trailing number

What does this MR do?

Currently, the docs say:

The username is set to project_{project_id}_bot, such as project_123_bot.

This is false:

Thus, an attempt to warn future readers so they look up the exact user name.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Not applicable, only documentation changed.

Security

Not applicable, only documentation changed.