fix misspelled word in comment

Change-Id: Ic56ac2be3df36fac555cb4d42ca5fd8b973ec1dd
GitHub-Last-Rev: 03c9ae0
GitHub-Pull-Request: #371
Reviewed-on: https://go-review.googlesource.com/c/163117
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: Madhu-1

oauth2.go

@@ -145,7 +145,7 @@ func SetAuthURLParam(key, value string) AuthCodeOption {
 //
 // Opts may include AccessTypeOnline or AccessTypeOffline, as well
 // as ApprovalForce.
-// It can also be used to pass the PKCE challange.
+// It can also be used to pass the PKCE challenge.
 // See https://www.oauth.com/oauth2-servers/pkce/ for more info.
 func (c *Config) AuthCodeURL(state string, opts ...AuthCodeOption) string {
 	var buf bytes.Buffer