fix opengraph tag

Author: StevenClontz

index.html

@@ -18,7 +18,7 @@
 <meta property="og:image" content="https://code4math.org/code4math.svg"/>
 <meta property="og:url" content="https://code4math.org/"/>
 <meta property="og:type" content="article">
-<meta property="og:site_name" content="Simple.css"/>
+<meta property="og:site_name" content="code4math.org"/>
 <meta property="og:description" content="Consortium of Digial Ecosystems for Mathematics."/>
 
 <!-- Twitter integration -->