Added docs for changing the default class name

[ArsalaBangash]

Motivation

I was having trouble explaining the prose class to my students, so I decided to look through the documentation to see if I could change the name.

I came across this issue, where @adamwathan last commented

It should work in the latest release, just haven't written release notes yet sorry.

Overview

I tried setting the class name to typography on a local nuxt build, and it worked perfectly, so I went ahead and created a PR to add this to the README documentation.

[adamwathan]

Thanks @ArsalaBangash!