tsnet: fix format string/argument mismatch in log output

Change-Id: Ia7291ea47a289baec6cc6013d63d2f248ae57d9e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f