[dns_record_set] change primary-backup -> failover in docs

[hkundag]

b/277747354

References to "Primary-Backup" in the dns_record_set documentation have been changed to "Failover".

Release Note Template for Downstream PRs (will be copied)

[github-actions]

Hello! I am a robot. Tests will require approval from a repository maintainer to run.

@c2thorn, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 2 files changed, 4 insertions(+), 4 deletions(-))
google-beta provider: Diff ( 2 files changed, 4 insertions(+), 4 deletions(-))

Missing test report

Your PR includes resource fields which are not covered by any test.

Resource: google_dns_record_set (31 total tests)
Please add an acceptance test which includes these fields. The test should include the following:

resource "google_dns_record_set" "primary" {
  routing_policy {
    primary_backup = # value needed
  }
}

[modular-magician]

Tests analytics

Total tests: 46
Passed tests: 45
Skipped tests: 1
Affected tests: 0

Click here to see the affected service packages

• dns

$\textcolor{green}{\textsf{All tests passed!}}$
View the build log

[c2thorn]

Suggested change

	Structure is [document below](#nested_primary_backup).
	Structure is [documented below](#nested_primary_backup).

I think this was correct originally?

[hkundag]

whoops, thanks for catching!

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))
google-beta provider: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

Missing test report

Your PR includes resource fields which are not covered by any test.

Resource: google_dns_record_set (31 total tests)
Please add an acceptance test which includes these fields. The test should include the following:

resource "google_dns_record_set" "primary" {
  routing_policy {
    primary_backup = # value needed
  }
}

[modular-magician]

Tests analytics

Total tests: 46
Passed tests: 45
Skipped tests: 1
Affected tests: 0

Click here to see the affected service packages

• dns

$\textcolor{green}{\textsf{All tests passed!}}$
View the build log

[c2thorn]

LGTM
Note: I've removed the changelog note as documentation-only changes are not usually included.