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>

api.md

@@ -1336,8 +1336,8 @@ It holds the capabilities specified in the request and can no longer be retrieve
 
 ``` jsonc
 {
-  "id":           "XXXX456CNTRL",
-  "key":          "tskey-k123456CNTRL-abcdefghijklmnopqrstuvwxyz",
+  "id":           "k123456CNTRL",
+  "key":          "tskey-auth-k123456CNTRL-abcdefghijklmnopqrstuvwxyz",
   "created":      "2021-12-09T23:22:39Z",
   "expires":      "2022-03-09T23:22:39Z",
   "revoked":      "2022-03-12T23:22:39Z",