Mark dockershim KEP as implemented

Signed-off-by: Davanum Srinivas <[email protected]>

Author: dims

keps/sig-node/2221-remove-dockershim/kep.yaml

@@ -5,7 +5,7 @@ authors:
   - "@dims"
 owning-sig: sig-node
 participating-sigs:
-status: implementable
+status: implemented
 creation-date: 2020-09-14
 reviewers:
   - "@yujuhong"