image/gif: fix typo in the comment (io.ReadByte -> io.ByteReader)

Fixes #46967

Change-Id: I66e69c70b74e904623e8ca854562d255692b2143
Reviewed-on: https://go-review.googlesource.com/c/go/+/331649
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Carlos Amedee <carlos@golang.org>

Author: uji

src/image/gif/reader.go

@@ -116,7 +116,7 @@ type decoder struct {
 // consumed when checking that the blockReader is exhausted.
 //
 // To avoid the allocation of a bufio.Reader for the lzw Reader, blockReader
-// implements io.ReadByte and buffers blocks into the decoder's "tmp" buffer.
+// implements io.ByteReader and buffers blocks into the decoder's "tmp" buffer.
 type blockReader struct {
 	d    *decoder
 	i, j uint8 // d.tmp[i:j] contains the buffered bytes