Fix misformatted GFM Table in BackendService

The markdown renderer for our proto API fails because of the line break.
It appears to either be a bug with the tool, or how we're using the tool.

See https://github.com/googleforgames/open-match-docs/pull/187

> Drat. I'd seen direct commits to this file as part of doc updates in the file history, but there wasn't a mention in the few commits I looked at (or the file itself) of auto-generation.
>
> We might want to remove the 'Edit this Page' link from the page, or all autogenerated pages if any others exist, since that is I got here in the first place.
>
> If I find time later, I might come back and look into the tool, but I don't even have an Open Match source checkout, so this is pretty low on my priority list.
>
> I don't see an issue in their tracker for the newlines in the type description, but I suspect pseudomuto/protoc-gen-doc#274 is related.
>
> I do see pseudomuto/protoc-gen-doc#282 which might be related to why the Scalar Value Types table fails to render as a table on the site, but looks fine in the GitHub Markdown preview. It might be that the HTML renderer for the site isn't using expecting whatever flavour of Markdown protoc-gen-doc outputs.
>
> Thanks anyway.