Fix typo in Intellij code style documentation

[Ahmed-Khaled-dev]

Removed the "At Java" part from the Code folding section in the Developer documentation, since it was a bit misleading to me 😅

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (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 developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

Thank you for your input. We made the text even more explicit at #10945.

[koppor]

We hope, that text appears also more clear to you 🎉