encoding/json: fix format string in the Fuzz func

Currently test build fails with:

$ go test -tags=gofuzz encoding/json
encoding/json/fuzz.go:36:4: Println call has possible formatting directive %s
FAIL	encoding/json [build failed]

Change-Id: I23aef44a421ed0e7bcf48b74ac5a8c6768a4841b
Reviewed-on: https://go-review.googlesource.com/c/go/+/190698


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>