Correct provider_reference doc

[shuyama1]

fixes hashicorp/terraform-provider-google#20765

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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 ( 1 file changed, 2 insertions(+), 2 deletions(-))
google-beta provider: Diff ( 1 file changed, 2 insertions(+), 2 deletions(-))