docs: fix incorrect A2A server instruction link in contact sample
This pull request addresses Issue #257, where the link for the A2A server instructions in the contact client sample was leading to an unhelpful directory view rather than the actual setup instructions.
The Problem: In samples/client/lit/contact/README.md, the "A2A server" link pointed to the contact_lookup folder. While not a literal 404, it forced users to hunt for the setup steps, which felt "broken" to new users.
The Fix: I have updated the link to use a direct relative path to the specific README.md file within the agent directory. This ensures that when a user clicks the link, they immediately see the:
Prerequisites (Python, UV)
API key setup instructions
Command to run the server (uv run .)
Related Issue Fixes #257