crypto/cipher: fix typo in example comment

LGTM=josharian
R=golang-codereviews, josharian
CC=golang-codereviews
https://golang.org/cl/110330043

src/pkg/crypto/cipher/example_test.go

@@ -277,7 +277,7 @@ func ExampleStreamWriter() {
 	}
 
 	// Note that this example is simplistic in that it omits any
-	// authentication of the encrypted data. It you were actually to use
+	// authentication of the encrypted data. If you were actually to use
 	// StreamReader in this manner, an attacker could flip arbitrary bits in
 	// the decrypted result.
 }