Update a link for version 2.1.3

Author: komainu8

index.md

@@ -54,7 +54,7 @@ sections:
       - title: "Language Spec"
         description: "Scala's formal language specification."
         icon: "fa fa-book"
-        link: https://scala-lang.org/files/archive/spec/2.12/
+        link: https://scala-lang.org/files/archive/spec/2.13/
 
   - title: "Scala Evolution"      
     links: