update basic usage code block in readme.md with correction

[ahmedh01]

There a slight mistake in the basic usage code in the instructions found in the readme.

In the import statement for main.js, it says:

import VueYouTubeIframe from 'vue-youtube-iframe';

However, this will not work. It in-fact needs to explicitly mention @techassi/, so the import statement is going to be:

import VueYouTubeIframe from '@techassi/vue-youtube-iframe'

^ will work.

[Techassi]

Thanks for pointing out this small mistake. You are 100% correct.

I updated the README in the latest commit edb063c