set unitCount=1 for ThumbnailOffset

[sdkawata]

fix #70

This pull request fixes a bug where certain tools (such as exiftool) raised warnings and failed to extract thumbnails properly when updating EXIF tags using this library. Upon investigation, setting unitCount for ThumbnailOffset to 1 resolves the issue.

Here's the code snippet I use to update EXIF tags:

package main

import (
	"io"
	"os"

	jpegstructure "github.com/dsoprea/go-jpeg-image-structure/v2"
)

func main() {
	f, err := os.Open("./input.jpg")
	if err != nil {
		panic(err)
	}
	imageBytes, err := io.ReadAll(f)
	if err != nil {
		panic(err)
	}
	jmp := jpegstructure.NewJpegMediaParser()
	intfc, err := jmp.ParseBytes(imageBytes)
	if err != nil {
		panic(err)
	}
	sl := (intfc).(*jpegstructure.SegmentList)
	rootIb, err := sl.ConstructExifBuilder()
	if err != nil {
		panic(err)
	}
	err = sl.SetExif(rootIb)
	if err != nil {
		panic(err)
	}
	wf, err := os.Create("./result.jpg")
	if err != nil {
		panic(err)
	}
	err = sl.Write(wf)
	if err != nil {
		panic(err)
	}
}