Electron-packager URL update

[Evo Stamatov]

URL in config/electron.js is coming with 'Page Not Found':
https://simulatedgreg.gitbooks.io/electron-vue/content/docs/building_your_app.html

URL can be found at:

electron-wrapper/template/config/electron.js

Line 5 in 9578a1c

 // Docs: https://simulatedgreg.gitbooks.io/electron-vue/content/docs/building_your_app.html 

Should redirect to (or should be) one of the following:

• https://simulatedgreg.gitbooks.io/electron-vue/en/using-electron-packager.html
• https://simulatedgreg.gitbooks.io/electron-vue/ja/using-electron-packager.html
• or as previously: https://simulatedgreg.gitbooks.io/electron-vue/en/building_your_app.html (or the ja alternative)
• or even https://github.com/SimulatedGREG/electron-vue/blob/master/docs/en/using-electron-packager.md (or the ja alternative)

Because of the above decision-making I'm not making a pull request.