playground: replace tinyurl with another URL shortener

[VladimirAlexiev]

(Don't know any better repo to report https://json-ld.org/playground/ problems. @msporny @gkellogg could you please dispatch?)

TinyURL has gone commercial:

Have 14 days of a TinyURL Subscription on us!
We want to treat you to a free trial of TinyURL's paid features. Just click the button below to sign up and avail of features like:
Once you’ve signed up, you have 14 whole days to take your link management and branding to the next level.

Please replace it with some other shortener service.

[msporny]

Transferring issue to json-ld/json-ld.org#838