encoding/xml: fix format in test message Found by go vet.
encoding/xml: fix format in test message
Found by go vet.

LGTM=crawshaw
R=golang-codereviews, crawshaw
CC=golang-codereviews
https://codereview.appspot.com/100510044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang