readme: change playground url

[av-elier]

Hi. Long time ago I added playground url. It was hosted on heroku. But heroku shutting down free dynos. Therefore, I moved playground project to my own domain and host.

Old URL will stop working by the end of november.

[lightbend-cla-validator]

Hi @av-elier,

Thank you for your contribution! We really value the time you've taken to put this together.

We see that you have signed the Lightbend Contributors License Agreement before, however, the CLA has changed since you last signed it. Please review the new CLA and sign it before we proceed with reviewing this pull request:

https://www.lightbend.com/contribute/cla

[av-elier]

I just signed a CLA. Bot email asked me to comment this.

[ennru]

CLA checked manually.