docs: fixed `yarn add -d`

[romanonthego]

incorrect casing of -d (note lowsace d) was causing yarn to fail when copy-pasting code from docs;

ref to yarn api

Context

In Getting Started section yarn add script have incorrect casing what causes yarn to fail when copy-pasting code from docs;

While very easy to fix - it could be a little annoying and can confuse newcomers.

Demo

This is a simple docs typo fix, no demo needed (I think?..)

Checklist

• [x] I've read CONTRIBUTING.md
• [x] Test everything manually
• [x] Write a good PR description