Fix a typo in CONTRIBUTING.md

[sitegui]

Hello Nextcloud Android team o/

I've started studying the codebase and I've noticed a small typo in the markdown of CONTRIBUTING.md that, causing part of the file to be rendered incorrectly.

🖼️ Screenshots

🏚️ Before

https://github.com/nextcloud/android/blob/448b459165b78f376a7bfb4db9a8562daa310938/CONTRIBUTING.md#testing

🏡 After

https://github.com/sitegui/nextcloud-android/blob/978b49eb281efca974355c4d197cc868e6c977b1/CONTRIBUTING.md#testing

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔙 Backport requests are created or not needed: /backport to stable-xx.x
• 📅 Milestone is set
• 🌸 PR title is meaningful (if it should be in the changelog: is it meaningful to users?)

🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

[sitegui]

Just to give more context: I'm also having problems with the auto-upload feature.

I saw the call for help in #16550, so I would like to use this as an excuse to learn more about the internals of this project and maybe help with the fixes. 🤓

Do you have the auto-upload architecture documented somewhere?
I guess I'll write one for me to help understanding it. I can share it later, if you feel that it's interesting to have it on the codebase.