fix(ibm_is_image): 404 error fix on datasource

[ujjwal-ibm]

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Relates OR Closes #0000

Output from acceptance testing:

% make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISImageDataSource_id404'
=== RUN   TestAccIBMISImageDataSource_id404
--- PASS: TestAccIBMISImageDataSource_id404 (2.92s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     4.615s

[ujjwal-ibm]