fix a small typo

eBook/07.5.md

@@ -31,7 +31,7 @@ func AppendByte(slice []byte, data ...byte) []byte {
 	m := len(slice)
 	n := m + len(data)
 	if n > cap(slice) { // if necessary, reallocate
-		// allocate doublke what's needed, for future growth.
+		// allocate double what's needed, for future growth.
 		newSlice := make([]byte, (n+1)*2)
 		copy(newSlice, slice)
 		slice = newSlice