The charset attribute on the script element is obsolete

I am trying to embed a follow me twitter button on my website but Html checker says;

Warning: The charset attribute on the script element is obsolete. From line 21, column 141; to line 21, column 216

eacher</a><script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></scri

I copied the link straight from twitter developer so I don't know why this error.