fixed the importer for is_instance for volumes import

[uibm]

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Relates OR Closes #0000

Output from acceptance testing:

% make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISInstance_VolumeAttachment'
=== RUN   TestAccIBMISInstance_VolumeAttachment
--- PASS: TestAccIBMISInstance_VolumeAttachment (259.01s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     260.781s