go.mod, ssh/tailssh: fix ImplictAuthMethod typo

Fixes #5745

Change-Id: Ie8bc88bd465a9cb35b0ae7782d61ce96480473ee
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