readme: npm i -> npm install

Although `npm i` is shorter to type, having it in the readme adds some unnecessary complexity. I had to stop and think to figure out what the command actually did. As all other projects I've come across uses `npm install`, I think it's better to follow an accepted convention and show the longer command.

For people that copy and pastes it won't make a difference, and people that type it out can still use the sort version.