api.md: update example auth key value

example was missing the "-auth" type in the key prefix, which all new
keys now contain.  Also update key ID to match the full key, and fix
indenting of closing braces.

Signed-off-by: Will Norris <will@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f