docs: removed duplicate `git_ssh_url` field from build event example

What does this MR do?

While implementing the GitLab web hooks in rust I noticed that one of the examples fails parsing since it contained duplicate keys. This MR removes the duplicate field. The change only touches one line of documentation.

I guess most of the acceptance criteria below aren't required?

What are the relevant issue numbers?

There is no issue for this change.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides