(trivial) Examples using Bored API need a small update

[sisyfus]

• www.boredapi.com appears to have moved to bored-api.appbrewery.com
• The line in the Activity factory:
  price: json['price']! as double, throws when price is an integer. I fixed it by changing it to:
  price: (json['price']! as num).toDouble(),

[Luckey-Elijah]

Even https://bored-api.appbrewery.com/ returns with "Too many requests, please try again later." quite frequently.

Should docs be rewritten to use a more stable/popular API? (which doesn't make this change nearly as "trivial")

[rrousselGit]

If you have a public API you want to propose as a replacement, sure!

The change shouldn't be too hard.