Fix: correct assertion message in StorageServiceTest for storage dir validation

[nanna3210]

Description of what I changed

Corrected the expected error message in StorageServiceTest#getDataShouldFailIfKeyTriesToAccessFilesOutsideStorageDir to match the actual exception message and ensure test passes.

Issue I worked on

No existing issue. Found this test failure while building the project.

Checklist: I completed these to help reviewers :)

• My IDE is configured to follow the code style of this project.
• I have added tests to cover my changes. (Test was already present; just fixed the assertion)
• I ran mvn clean package right before creating this pull request and added all formatting changes to my commit.
• All new and existing tests passed.
• My pull request is based on the latest changes of the master branch.

[atulyadav745]

Hey @nanna3210 Thank you for working on this,
I would highly encourage to create an issue on JIRA before actually making a PR and mention the issue number in your PR.
Title for Pr should be <issue_number>_what_changes_your_PR_proposes
I would suggest you to go through the OpenMRS contribution Guidelines
Thanks!