Fix some typos

[koppor]

While coding, I saw that IntelliJ marks some code comments as typos. This PR fixes that.

Additionally, one small code change to keep the variable declaration close to the usage.

• 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 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.

[Siedlerchr]

I remember that we had the policy that one shouldn't just contribute random code style fixes, but instead improving the code quality while fixing a bug or implementing a feature... :-P

[koppor]

I wanted to separate the code style things from the other PR as we discuss for a long time to gain acceptance. - I thought that #8158 takes longer to merge.