About readme.md's "初始账户" section, change elastic user default password

[VAllens]

In addition to changing the password in the docker-compose.yml file, we also need to change the password for the metricbeat/metricbeat.yml file.

Otherwise you can't work properly.

[VAllens]

It seems that all the 123456 passwords should be changed

[genox-wang]

Thanks for your suggestion, it's my negligence. I will change readme. yes, password should be changed