compress/flate: use io.ByteReader in docs

We did this elsewhere when io.ByteReader was added
but forgot this one.

R=golang-dev, khr
CC=golang-dev
https://golang.org/cl/43480052

src/pkg/compress/flate/inflate.go

@@ -180,7 +180,7 @@ func (h *huffmanDecoder) init(bits []int) bool {
 // the NewReader will introduce its own buffering.
 type Reader interface {
 	io.Reader
-	ReadByte() (c byte, err error)
+	io.ByteReader
 }
 
 // Decompress state.