crypto/cipher: fix typo from last change

s/encrypt/decrypt/

The text is unsafe to cut and paste...

Change-Id: Iab19ddf8182d087e9a4b4d34a9eeabd1d2aa02d6
Reviewed-on: https://go-review.googlesource.com/18104Reviewed-by: Rob Pike <r@golang.org>

src/crypto/cipher/gcm.go

@@ -37,7 +37,7 @@ type AEAD interface {
 	// value passed to Seal.
 	//
 	// The ciphertext and dst may alias exactly or not at all. To reuse
-	// ciphertext's storage for the encrypted output, use ciphertext[:0] as dst.
+	// ciphertext's storage for the decrypted output, use ciphertext[:0] as dst.
 	Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error)
 }