fix ibm_container_storage_attachment import

[ceccone]

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Relates OR Closes #0000

Output from acceptance testing:

$ export TESTARGS="-run TestAccIBMContainerVpcClusterWorkerVolumeAttachment_Basic"
$ make testacc
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccIBMContainerVpcClusterWorkerVolumeAttachment_Basic -timeout 700m
...
=== RUN   TestAccIBMContainerVpcClusterWorkerVolumeAttachment_Basic
--- PASS: TestAccIBMContainerVpcClusterWorkerVolumeAttachment_Basic (1526.43s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/kubernetes	1526.458s