Fix: rm anonymous functions in .pre hooks

In official mongoose docs everywhere used `function()` instead of `() => {}`
https://mongoosejs.com/docs/middleware.html#pre

It simply confused me when I've copied code from example and tryed to reach `this` in `.pre('save', () => console.log(this))` there were module itself bcs it loses context. I've spend couple of hours to figure out the issue - it's not that obvious. So my purpose to put `function() {}` there, bcs almost in all cases users will try to do something with document passed to the hook.

Author: BlindBadCat

content/techniques/mongo.md

@@ -265,7 +265,7 @@ Middleware (also called pre and post hooks) are functions which are passed contr
         name: Cat.name,
         useFactory: () => {
           const schema = CatsSchema;
-          schema.pre('save', () => console.log('Hello from pre save'));
+          schema.pre('save', function() { console.log('Hello from pre save') });
           return schema;
         },
       },
@@ -286,11 +286,11 @@ Like other [factory providers](https://docs.nestjs.com/fundamentals/custom-provi
         imports: [ConfigModule],
         useFactory: (configService: ConfigService) => {
           const schema = CatsSchema;
-          schema.pre('save', () =>
+          schema.pre('save', function() {
             console.log(
               `${configService.get('APP_NAME')}: Hello from pre save`,
             ),
-          );
+          });
           return schema;
         },
         inject: [ConfigService],