Fix last-updated typo for plugins KEP

There was a typo in the last update to the plugins KEP as part of
commit: 1d0bf29
Clearly the year should have been 2019 and not 2010.

Signed-off-by: Marc Khouzam <[email protected]>

Author: marckhouzam

keps/sig-cli/2379-kubectl-plugins/kep.yaml

@@ -15,7 +15,7 @@ approvers:
   - "@soltysh"
 editor: juanvallejo
 creation-date: 2018-07-24
-last-updated: 2010-02-26
+last-updated: 2019-02-26
 status: implemented
 see-also:
   - n/a