Fix a broken link to Handshake Protocol Summary

[handshake-enthusiast]

The Handshake Protocol Summary page was renamed and moved to another location. See:

• https://github.com/handshake-org/handshake-org.github.io/commits/master/src/protocol/summary.md
• https://github.com/handshake-org/handshake-org.github.io/commits/7b30d6f47f200feb96b9c31da209bf3e48fb1c72/src/guides/protocol.md?browsing_rename_history=true&new_path=src/protocol/summary.md&original_branch=master