Clean up markdown for signed_url a bit

[rileykarson]

Upstreams hashicorp/terraform-provider-google#4652

Release Note Template for Downstream PRs (will be copied)

[googlebot]

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[rileykarson]

This modifies a third_party directory where the CLA does not apply.

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, 1d0566c.

Pull request statuses

No diff detected in terraform-google-conversion.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#1250
depends: hashicorp/terraform-provider-google#4656