thousands -> millions

We do have 3 million weekly downloads on npm, so I think "thousands" was quite an understatement at this point.

assets/templates/header-main.html

@@ -4,7 +4,7 @@ <h1><a href="index.html"><img src="assets/logo.svg" alt="Prism" /></a></h1>
 
 <p>
 	Prism is a lightweight, extensible syntax highlighter, built with modern web standards in mind.
- 	It’s used in thousands of websites, including some of those you visit daily.
+ 	It’s used in millions of websites, including some of those you visit daily.
 </p>
 
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