devapp/owners: add myself to encoding/json

Done more stuff there recently and it needs attention, so it seems like
a right fit.

Change-Id: I6344ccc5da83c0cfecd7a7f2ecc1399497e0de03
Reviewed-on: https://go-review.googlesource.com/131285
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/devapp/owners/table.go b/devapp/owners/table.go
index bd07fd1..61054ec 100644
--- a/devapp/owners/table.go
+++ b/devapp/owners/table.go
@@ -232,7 +232,7 @@
 	},
 	"go/src/encoding/json": {
 		Primary:   []Owner{rsc},
-		Secondary: []Owner{joetsai, bradfitz},
+		Secondary: []Owner{joetsai, bradfitz, mvdan},
 	},
 	"go/src/encoding/xml": {
 		Primary: []Owner{rsc},