derp: fix omitted word in comment

Fix comment just added in 38f236c725.

Updates tailscale/corp#23668
Updates #cleanup

Change-Id: Icbe112e24fcccf8c61c759c631ad09f3e5480547
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