Replaces dashboard help link in project helper

What does this MR do?

Replaces the dashboard help link in projects_helper.rb to be inline with the new docs location.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• Separation of EE specific content

Closes #12601 (closed)