examples: use a valid DNS name

This prevents a gnutls server from sending an unexpected message alert due to invalid DNS name encoding, if the example is not modified.

Checklist

• Code modified for feature

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code