Fix interactive example

[yuroitaki]

The interactive example is broken due to the use of a deprecated endpoint of the hosted notary server.

Replace the endpoint with a different endpoint using a different domain as the hosted notary server only has versioned endpoints now (which will always be deprecated in the future since we only maintain the latest 3 versions). This also means that the verification logic in the code has to be changed.

[heeckhau]

@yuroitaki Let's replace this with another TLS-server, using the notary server here is too confusing.
Let's use https://example.com/ instead

[heeckhau]

Fixed in #552