Remove "online" prefix in the expected file URLs to fix Springer fetcher tests

[Toromtomtom]

This addresses two of the errors in the fetcher tests as run by the CI (see #6369). Examples for the failures are here and here.

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.

[Siedlerchr]

Ah yes that was recently changed. Thanks for fixing this

[calixtus]

Yes, thanks, overlooked them. Thank you very much.