docs: unify deploy script name

[miyaoka]

I was a bit confused when deploying according to the guide.
So I think it would be better to use the same script name.

on getting-started.md

{ "scripts": { "docs:dev": "vuepress dev docs", "docs:build": "vuepress build docs" } } 

on deploy.md

{ "scripts": { "build-docs": "vuepress build docs" } }