English grammar and typo fixes in CONTRIBUTING.md

[bharatr21]

---

• Change in CHANGELOG.md described
• Tests created for changes
• Manually tested changed features in running JabRef
• Screenshots added in PR description (for bigger UI changes)
• Ensured that the git commit message is a good one

[koppor]

Thank you for your quick update.

Could you also fix the linting isuses in the markdown file? (Think empty line after the heading is missing) --> https://github.com/DavidAnson/vscode-markdownlint

[LinusDietz]

Looks good to me now. Thank you for these improvements. If you like continuing working on JabRef, have a look at the issues as well (;

[koppor]

I did not make myself clear. I just wanted to have the empty line removed. I like the new text.

I will revert the revert, remove the empty line and then merge.

[koppor]

Merged in e393a10