control/controlclient: add missing word in comment

Found by review.ai.

Updates #cleanup

Change-Id: Ib9126de7327527b8b3818d92cc774bb1c7b6f974
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