cmd/tailscale/cli: fix inconsistency between serve text and example command

Use the same local port number in both, and be more precise about what
is being forwarded

Signed-off-by: Mihai Parparita <mihai@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