types/ipproto: fix a typo in a doc comment

Updates #cleanup

Change-Id: I5d046c6997770602336bda49d7a6fab41f76401c
Signed-off-by: Alex Chan <alexc@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