Update content/techniques/logger.md

Author: kamilmysliwiec

content/techniques/logger.md

@@ -155,7 +155,7 @@ import { Logger, Injectable } from '@nestjs/common';
 
 @Injectable()
 class MyService {
-  logger = new Logger(MyService.name);
+  private readonly logger = new Logger(MyService.name);
 
   doSomething() {
     this.logger.log('Doing something...');