Add missing type argument to docs for google_dns_record_set data source

[SarahFrench]

Closes hashicorp/terraform-provider-google#13586

The type argument that wasn't listed in the docs is present in the examples in the docs, and also present in tests for the data source.

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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.

Terraform GA: Diff ( 1 file changed, 2 insertions(+))
Terraform Beta: Diff ( 1 file changed, 2 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[trodge]

LGTM