Typo and incorrect wording in contribute.md

[frica]

Hello,

Sorry to disturb for such a small issue. I came across your project today and while exploring the website, I discovered a typo that bothered me 😅

On the contribute page it's written: GitJournal is an [Open Souce project](https://github.com/GitJournal/GitJournal), and can always use the help. instead of Open Source.

Moreover the term ´Open Source´ should only apply for the Open Source Initiative.
As an adjective it should be hyphenated like : open-source (see recommendation).

If you think it's relevant I made a quick patch or I can submit a PR if you prefer.

fix_typo_+_correct_term.patch

Regards,

Fabien