doc: update import path in readme

Author: Bart Huijgen

readme.md

@@ -4,7 +4,7 @@
 
 ```ts
 import { Kysely } from "https://cdn.jsdelivr.net/npm/[email protected]/dist/esm/index-nodeless.js";
-import { PostgresDialect } from "[not hosted yet]";
+import { PostgresDialect } from "https://deno.land/x/kysely_postgres/mod.ts";
 
 const db = new Kysely<{}>({
   dialect: new PostgresDialect({