crypto/cipher: Fix comment punctuation

Change-Id: I7dc086a87d28ab847288eed13f719421420cd004
Reviewed-on: https://go-review.googlesource.com/c/go/+/686997
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/crypto/cipher/cfb.go

@@ -86,7 +86,7 @@ func NewCFBDecrypter(block Block, iv []byte) Stream {
 func newCFB(block Block, iv []byte, decrypt bool) Stream {
 	blockSize := block.BlockSize()
 	if len(iv) != blockSize {
-		// stack trace will indicate whether it was de or encryption
+		// Stack trace will indicate whether it was de- or en-cryption.
 		panic("cipher.newCFB: IV length must equal block size")
 	}
 	x := &cfb{