[d1] Fix command to generate SQL for User table

[yangshun]

Summary

Running the command in the doc shows this error:

npx prisma migrate diff --from-empty --to-schema-datamodel ./prisma/schema.prisma --script --output migrations/0001_create_user_table.sql
Loaded Prisma config from prisma.config.ts.

Error: 
`--to-schema-datamodel` was removed. Please use `--[from/to]-schema` instead.

This PR updates the docs.

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Documentation checklist

• The change adheres to the documentation style guide.