oauth2: fix typo
oauth2: fix typo

Change-Id: Ic39a63c50a042ab836bbc496db9aea7fb52870d2
Reviewed-on: https://go-review.googlesource.com/1652
Reviewed-by: Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

pengwynn/oauth2-1,freeformz/oauth2,steveruckdashel/oauth2,golang/oauth2,lancecarlson/oauth2,dsriram/oauth2,tpiha/oauth2,walmartlabs/oauth2,paul-at-start/oauth2,equinux/oauth2,drewwells/oauth2,awormuth/oauth2,cllu/oauth2,jgalilee/oauth2,qtzheng/oauth2,cw2018/oauth2,EagerIO/oauth2,broady/oauth2,helojo/oauth2,Kuzmin/oauth2,zj8487/oauth2,dbmedialab/golang-oauth2,danielkarch/oauth2,hoysoft/oauth2,hoonmin/oauth2,djalal/oauth2,flowlo/oauth2,jboeuf/oauth2,wmydz1/oauth2,heshed/oauth2