Fix missing quotation mark

Author: illuhad

index.html

@@ -5,7 +5,7 @@
   overlay_color: "#5e616c"
   overlay_image: /assets/images/splash.png
   cta_label: "<i class='fa fa-download'></i> Get it"
-  cta_url: "https://github.com/AdaptiveCpp/AdaptiveCpp
+  cta_url: "https://github.com/AdaptiveCpp/AdaptiveCpp"
   caption: 
 excerpt: 'All things AdaptiveCpp.'
 feature_row: