Fixing incorrect encoding of an HTML entity within an HTML attribute.

The source of an annoying pub build warning:

line 9, column 1 of package:paper_elements/src/font-roboto/roboto.html: Named entity expected. Got none.
<link href='//fonts.googleapis.com/css?family=RobotoDraft:regular,bold,italic,thin,light,bolditalic,black,medium&lang=