openpgp: fix a typo in a comment

Change-Id: I9fe23643ae644c4cc4a36b14ad6efe99197c8beb
Reviewed-on: https://go-review.googlesource.com/29959
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

openpgp/read.go

@@ -50,7 +50,7 @@ type MessageDetails struct {
 	// If IsSigned is true and SignedBy is non-zero then the signature will
 	// be verified as UnverifiedBody is read. The signature cannot be
 	// checked until the whole of UnverifiedBody is read so UnverifiedBody
-	// must be consumed until EOF before the data can trusted. Even if a
+	// must be consumed until EOF before the data can be trusted. Even if a
 	// message isn't signed (or the signer is unknown) the data may contain
 	// an authentication code that is only checked once UnverifiedBody has
 	// been consumed. Once EOF has been seen, the following fields are