fix(docs): correct drizzle import in Supabase and Xata get-started

[emmayusufu]

Closes drizzle-team/drizzle-orm#3161.

The first connection example on the Supabase and Xata get-started pages has two problems:

• It imports drizzle from drizzle-orm, which doesn't export it, so you get Module '"drizzle-orm"' has no exported member 'drizzle'.ts(2305).
• It calls drizzle('postgres-js', process.env.DATABASE_URL), but the postgres-js driver's drizzle takes a connection string (or client) as the first argument, not a dialect name.

Both pages use the postgres driver, so I changed them to import from drizzle-orm/postgres-js and pass the connection string directly, matching how the node-postgres get-started page already does it:

import { drizzle } from 'drizzle-orm/postgres-js'
const db = drizzle(process.env.DATABASE_URL!);

#3161 only mentions Supabase, but Xata had the identical broken snippet, so I fixed both. The later examples on these pages already use the correct drizzle-orm/postgres-js import, so this just brings the first one in line.