crypto/cipher: Fix typo in example comment

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/101510047

src/pkg/crypto/cipher/example_test.go

@@ -240,7 +240,7 @@ func ExampleStreamReader() {
 	}
 
 	// 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 output.
 }