jwt: added missing format specifier
jwt: added missing format specifier

Updates golang/oauth#151

Change-Id: I2422dade4d72aa4fc33d9ad922508d2793e4ee27
Reviewed-on: https://go-review.googlesource.com/14779
Reviewed-by: Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

danielkarch/oauth2,paul-at-start/oauth2,awormuth/oauth2,jboeuf/oauth2,dbmedialab/golang-oauth2,golang/oauth2,drewwells/oauth2,pengwynn/oauth2-1