Fix description for attribute import_custom_routes

[rileykarson]

Upstreams hashicorp/terraform-provider-google#6799

Release Note Template for Downstream PRs (will be copied)

[google-cla]

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[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=135139"

[rileykarson]

Changes are to a third_party directory where the CLA doesn't apply.

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccEndpointsService_basic You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=135145"