api: update next.txt for binary.AppendByteOrder interface

CL 386017 added new API for encoding/binary package.
This file was accidentally not updated in the same CL.

Updates #50601

Change-Id: Iefeb596ba04b8c6576cf0fe42030f658a5848832
Reviewed-on: https://go-review.googlesource.com/c/go/+/389636
Trust: Joseph Tsai <joetsai@digital-static.net>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

api/next.txt

@@ -0,0 +1,5 @@
+pkg encoding/binary, type AppendByteOrder interface { AppendUint16, AppendUint32, AppendUint64, String }
+pkg encoding/binary, type AppendByteOrder interface, AppendUint16([]uint8, uint16) []uint8
+pkg encoding/binary, type AppendByteOrder interface, AppendUint32([]uint8, uint32) []uint8
+pkg encoding/binary, type AppendByteOrder interface, AppendUint64([]uint8, uint64) []uint8
+pkg encoding/binary, type AppendByteOrder interface, String() string