fix: use backtick instead of link for CONTRIBUTING.md in template

The lychee link checker was validating the markdown link inside the
README template code block. Changed to backtick format since this
is template content, not an actual file reference.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: sjnims

plugins/plugin-dev/skills/marketplace-structure/examples/community-marketplace.md

@@ -126,7 +126,7 @@ A curated collection of high-quality Claude Code plugins.
 
 ## Contributing
 
-See [CONTRIBUTING.md](CONTRIBUTING.md) for submission guidelines.
+See `CONTRIBUTING.md` for submission guidelines.
 
 ## License