sample(mongoose): use HydratedDocument for define document interface

ref: nestjs/docs.nestjs.com#2517

Author: arkist

sample/06-mongoose/src/cats/schemas/cat.schema.ts

@@ -1,7 +1,7 @@
 import { Prop, Schema, SchemaFactory } from '@nestjs/mongoose';
-import { Document } from 'mongoose';
+import { HydratedDocument } from 'mongoose';
 
-export type CatDocument = Cat & Document;
+export type CatDocument = HydratedDocument<Cat>;
 
 @Schema()
 export class Cat {