docs: fix alt text and capitalization in documentation

[saibharath48]

• Fix incorrect alt text in README.md for mask and zone editor screenshot

• Capitalize 'Frigate' in audio_detectors.md for consistency

Proposed change

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code
• Documentation Update

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• UI changes including text have used i18n keys and have been added to the en locale.
• The code has been formatted using Ruff (ruff format frigate)

[netlify]

✅ Deploy Preview for frigate-docs ready!

Name	Link
🔨 Latest commit	e767324
🔍 Latest deploy log	https://app.netlify.com/projects/frigate-docs/deploys/695df715efa66c00088ed5f7
😎 Deploy Preview	https://deploy-preview-21551--frigate-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.