dev:ai script typo corrected in package.json

The dev:ai script in n8n's root package.json contained a typo that prevented developers from spinning up the AI development environment locally. The turbo filter was referencing @n8n/nodes-langchain instead of the correct @n8n/n8n-nodes-langchain package name. The fix corrects this one-character omission, restoring the script's functionality.

Developers working on n8n's langchain integration nodes can now run npm run dev:ai as intended to start the relevant services in parallel. The change is minimal—just a single character difference in a filter flag—but it unblocks the entire local AI development workflow.

This fix lives in the root package.json of the n8n monorepo.