Update middlewares.md

Type for the `next` parameter should be `NextFunction`
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/express/index.d.ts#L107
Even eslint gives an error: `Don't use `Function` as a type. The `Function` type accepts any function-like value.`

Author: notiv-nt

content/middlewares.md

@@ -23,11 +23,11 @@ You implement custom Nest middleware in either a function, or in a class with an
 ```typescript
 @@filename(logger.middleware)
 import { Injectable, NestMiddleware } from '@nestjs/common';
-import { Request, Response } from 'express';
+import { Request, Response, NextFunction } from 'express';
 
 @Injectable()
 export class LoggerMiddleware implements NestMiddleware {
-  use(req: Request, res: Response, next: Function) {
+  use(req: Request, res: Response, next: NextFunction) {
     console.log('Request...');
     next();
   }