Fix storage typo

[JeannedArk]

Only a typo fix

[google-cla]

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[lahirumaramba]

Thank you for your contribution, but the changes should be based off of the dev branch. https://github.com/firebase/firebase-admin-go/blob/master/CONTRIBUTING.md#want-to-submit-a-pull-request

Make sure to create all your pull requests against the dev branch. All development work takes place on this branch, while the master branch is dedicated for released stable code. This enables us to review and merge routine code changes, without impacting downstream applications that are building against our master branch.