fix misspelled word in comment

Change-Id: Ic56ac2be3df36fac555cb4d42ca5fd8b973ec1dd
GitHub-Last-Rev: 03c9ae0ae443b934d8c86de9529c0ec1ca7510a4
GitHub-Pull-Request: golang/oauth2#371
Reviewed-on: https://go-review.googlesource.com/c/163117
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/oauth2.go b/oauth2.go
index ec6ee00..428283f 100644
--- a/oauth2.go
+++ b/oauth2.go
@@ -145,7 +145,7 @@
 //
 // 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