oauth2: fix typo in comment

Change-Id: Ifc12ed36671d21e388c537c510a82be084e9ec99
GitHub-Last-Rev: a4da759
GitHub-Pull-Request: #720
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/580715
TryBot-Bypass: Cody Oss <codyoss@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>
Reviewed-by: Joedian Reid <joedian@google.com>

oauth2.go

@@ -393,7 +393,7 @@ func ReuseTokenSource(t *Token, src TokenSource) TokenSource {
 	}
 }
 
-// ReuseTokenSource returns a TokenSource that acts in the same manner as the
+// ReuseTokenSourceWithExpiry returns a TokenSource that acts in the same manner as the
 // TokenSource returned by ReuseTokenSource, except the expiry buffer is
 // configurable. The expiration time of a token is calculated as
 // t.Expiry.Add(-earlyExpiry).