docs(mongo): change the way to define the mongoose document interface

Mongoose introduced `HydratedDocument` from 6.0.13.

refs:
https://github.com/Automattic/mongoose/blob/6.7.0/docs/typescript.md#creating-your-first-document
Automattic/mongoose@8120c8f

Author: arkist

content/techniques/mongo.md

@@ -34,9 +34,9 @@ Let's define the `CatSchema`:
 ```typescript
 @@filename(schemas/cat.schema)
 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 {