Update example output for node inspect

commit 41d580c7a610d8111dc63353cbd94319ca854627 added
"Joined at" to the output of node inspect,
but I forgot that docs changes are needed.

This change updates the example output
of node inspect in the documentation.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/node_inspect.md

@@ -99,6 +99,7 @@ Example output:
     $ docker node inspect --pretty self
     ID:                     e216jshn25ckzbvmwlnh5jr3g
     Hostname:               swarm-manager
+    Joined at:              2016-06-16 22:52:44.9910662 +0000 utc
     Status:
      State:                 Ready
      Availability:          Active