oauth2: fix typo

Change-Id: I515f8897cc79c58a8a49df84ccddc5acd9536d87
GitHub-Last-Rev: 5acbebb
GitHub-Pull-Request: #616
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/459695
Run-TryBot: Cody Oss <codyoss@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cody Oss <codyoss@google.com>
Reviewed-by: Matt Hickford <matt.hickford@gmail.com>

oauth2.go

@@ -140,7 +140,7 @@ func SetAuthURLParam(key, value string) AuthCodeOption {
 //
 // State is a token to protect the user from CSRF attacks. You must
 // always provide a non-empty string and validate that it matches the
-// the state query parameter on your redirect callback.
+// state query parameter on your redirect callback.
 // See http://tools.ietf.org/html/rfc6749#section-10.12 for more info.
 //
 // Opts may include AccessTypeOnline or AccessTypeOffline, as well