Fix import statement in fontawesome documentation

[kavanpancholi]

Currently in Font-Awesome documentation, it says

import {ThumbsUp} from 'vue-rate-it/glyphs/thumbs-up';

But there is not ThumbsUp object in vue-rate-it/glyphs/thumbs-up file.

And it says

created() {
    // register the icon
    this.thumbsUp = ThumbsUp
},
data() {
    return{
        thumbsUp: '' // declare the icon
    } 
} 

But it doesn't require to set in created function. It will work just fine if we set ThumbsUp in data.
Like this:

data() {
    return {
        thumbsUp: ThumbsUp // declare the icon
    } 
} 

So this PR fixes that with simpler documentation