Readme logging into mysql container

I think readme is not correct when it comes to logging into mysql container.

I have to do docker ps and get sha of running container to get inside the container, the label doesnt work for some reason.
Neither mysql or mysqldb did not work.

```
docker-nginx-php-mysql|master⚡ ⇒ docker exec -it mysql bash

Error: No such container: mysql

docker-nginx-php-mysql|master⚡ ⇒ docker exec -it 89dcfc77a965 bash
root@89dcfc77a965:/# 
```

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