Sample: fix prebid js loading bug on sample html page

[And1sS]

There is a bug on sample html page:

prebid.js:12 
Uncaught TypeError: Cannot read properties of null (reading 'appendChild')
    at U (prebid.js:12:9881)
    at $.processQueue (prebid.js:12:99684)
    at prebid.js:631:11

This is caused by appendChild() method call (inside pbjs) on a DOM element that doesn't exist yet .

This can be fixed by simply by deferring javascript scripts loading, and that is what i did in this pr.