Add `defer` to the script tag in the usage section

[waldyrious]

This way the script waits until the DOM is ready to execute.

[dohliam]

@waldyrious Thanks for this PR!

It makes sense (in theory) to add the defer attribute here, but do you have any examples of pages that load slowly enough for this to make a difference? Also, if it makes sense to add it here, should we also be adding it to index.html as well?