fix docs for createInstance

[felixroos]

hi! got a little confused testing this sdk, as the readme contains:

const pluginInstance = await pluginFactory.createInstance(audioContext, {});

same in the wiki.

the actual call signature in the code is different:

sdk/src/WebAudioModule.js

Line 17 in 56d68a2

static createInstance(groupId, audioContext, initialState) {

so it seems the first argument was added after the docs were written? is the groupId just an arbitrary string?