docs: fix missing quote in collection sources glob pattern

[devgofer]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR fixes a small typo in the sources section of the documentation.
In the line:

• source: '** includes all files within the content directory and its subdirectories.

a closing quote was missing in the glob pattern example. It has been corrected to:

• source: '**' includes all files within the content directory and its subdirectories.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/content@3368

commit: 89d8e4d

[farnabaz]

Thanks