oauth2: fix typo in Token docs and clarify mutability of Token

Change-Id: I1093cffbd4b6fe25cd5b7cfb31e820b15fcfa41f
Reviewed-on: https://go-review.googlesource.com/2197
Reviewed-by: Burcu Dogan <jbd@google.com>

token.go

@@ -16,7 +16,7 @@ import (
 //
 // Most users of this package should not access fields of Token
 // directly. They're exported mostly for use by related packages
-// implementing derivate OAuth2 flows.
+// implementing derivative OAuth2 flows.
 type Token struct {
 	// AccessToken is the token that authorizes and authenticates
 	// the requests.